Is there any reason to place polarizer filter before (the first in stack) ND or/and ND graduated one (for example as we can see on Lee 105 mm ring)? Is there any technical reason to do that or it's a useability or marketing issue(s)?
The polarizer needs to be able to rotate independently of other filters. Square/rectangular system filters only have one other filter type, the graduated/split filters, that needs to be rotated at all. The entire filter holder can be rotated to accommodate the angle required for the split/graduated filter. But what do you do when the horizon (or whatever line is dividing the split/grad filter use) is different from the angle you need to set the polarizer to?
Note that the square filter catalog includes a polarizer. If you don't need to use a split/grad filter (whether that's a neutral density, colour correction or colour effect filter), you can use a square polarizer anywhere in the filter stack. If you intend to use a split/grad, then you need to mount the polarizer in such a way that it can be rotated independently. In the Lee 100mm kit case, that means attaching a filter ring to the front of the holder (or the front of the hood). Why not use a special mount for the split/grad? Because you need to change the position of the rectangular split/grad filter as well as the angle, so it needs to use a regular slot in the holder.
(In the past, a square filter holder with a worm-gear-driven polarizer was available. That may have been for the similar Cokin system; it was a long time ago and my memory ain't what it used to be. In any casse, that could be put anywhere in the stack, but there was a problem: it took forever to adjust, and was quite fragile.)
I can't think of a technical reason.
Other than I would tend to change the ND grad more often than remove the polariser when doing landscapes.
A polarizer filter is also a weak ND filter (roughly 1 stop). So you can just use it to have a little more reduction in brightness in case the ND filter is not sufficient.
Polarizer filters are different, but similar to ND filters. ND filters block all light equally, but polarizing filters block light for specific angles of the wave. In combination, the ND filter reduces the overall light equally and the circular can be used to further reduce specific light angles. So, for example, in an extremely bright scene, you might reduce the total light volume to, for example, get a dreamy water appearance. In addition, you might use the CP to ensure that you have no specular highlights on the water.
Combinations of circular polarizers are, nicely enough, also used to create variable ND filters by adjusting both to block various angles of light. So, stacking them with ND filters creates a base ND block coupled by some variable amount according to need. Stacking filters of these types allows for some pretty fine control of the amount of light let through to the sensor and ultimately makes for some really powerful long exposure options in otherwise too bright scenes.
Question changed a little, but added clarity... The reason to have it at the beginning of the stack is to allow for rotation. Filter holders such as Lee or Cokin have CPs that fit their holders and are still round so that they can be rotated, which you need to do independent of any other filters and the rotation of the whole holder.
Putting the ND filter last, or closest to the lens, will cut down on reflections which result in lens flare.
Where-ever there is a surface in front of the lens (such as a filter), there is the potential for any small unwanted reflections bouncing from the lens to be reflected back into the lens and form lens flare. Here is an example of the type of lens flare you get from a UV filter. And this filter has an anti-reflective coating (from the green tinge, you may recognise it as a single-coated filter).
All glass surfaces will cause this, including the elements in the lens itself, but the elements inside the lens typically have very good anti-reflective coatings. You will get lens flare from the lens itself if there are strong light points in the picture, but you'll get more, and in some cases much more noticeable ones, if you have a filter in front of the lens.
An ND filter, especially one that's 3 stops or more, naturally reduces reflections simply because it reduces all light. Any reflections that pass through the ND filter will be reduced by 3 stops on the way out of the filter, then if there are further reflections from the outer surface of the ND filter or from subsequent filters, these will again be reduced by a further 3 stops as they travel back inwards. This reduces reflections by 64 times, for those reflections which pass through the ND filter twice.
If you have other filters between the ND filter and the lens, there is potential for reflections from the lens to bounce off these filters and back into the lens immediately without passing through the ND filter.
(This is the same reason why a lot of ND filters with 3 stops or more (8x or more) attenuation are not coated; they don't need it so much as the light attenuation of the filter itself is as good at reducing reflections.)
So to summarise, in so far as you get lens flare by having multiple filters, you'll get less lens flare by having the ND filter the closest one to the lens.
The question is semantically not clear. "Before" can mean "first", opposite of "after". "Before" can mean "in front", opposite of "behind". And it depends what exactly comes first, i.e. closest to the front element.
But my experience is this: When I use an ND, for example a 10-stopper, then first I put the CPL onto the front of the lens, so I can lock M focus the subject. Then I put the ND on top of the CPL, and I can't see a thing but I take a shot.
If I would first put the ND 10-stopper, then I wouldn't see a thing, thus unable to focus correctly. To put a CPL on top of the ND would not enable me to properly set (rotate) the CPL into position because I don't see a thing.
Simple as that.
You need to put the polarizing filter closest to your lens first so that you can see its effect as you rotate it. Once you put on the ND filter(s), it may be too dark to select the polarization.
Is there any reason to place polarizer filter before ND graduated one?
Exactly the same reason why placing a polarizer before a front element (of a lens): the longer exposure time results in the same amount of light reaching the sensor anyway.