IPTC is the International Press Telecommunications Council.
Fields in IPTC metadata are primarily intended to provide headline, caption, content details, rights holder(s), and source information for images when published by news organizations. If you're not submitting the images for publication by a news organization or any other entity that automates the titles and captions of images based on the content of the IPTC fields, you can use them however you wish.
The IPTC standard is mostly about the technical aspects of how it is implemented within the metadata of image files. For more specific usage guides as to what is and is not acceptable usage of particular fields, you can probably check out the guidelines of specific news agencies, such as the Associated Press or Reuters. There are a few differences between various news agencies with regard to what information a specific agency wants (and does not want) in a particular field.
Many require the use of a Controlled Vocabulary for many fields. A CV is a list of codes or words for a particular field when they don't want free text or any other terms not on their list of options to be placed in those fields. The IPTC itself does not have a list of CV terms for most fields. They leave that up to the organizations that use IPTC data to define the terms for themselves.
The "Title" field, as described in the IPTC Photo Metadata User Guide
- 2017 Edition is intended to be a unique identifier for the benefit of the creator of an image or for an organization that archives the image.
A short human readable reference for the image. It can be a text reference or a numeric reference, and serves primarily as an identifier. It has been used by photographers for their image filename, though since about 2008 IPTC now provides specific fields for image IDs like Digital Image GUID or Registry Entry (those wishing to, can use the Registry Entry. The Title field should not be confused with the Headline field which is a short descriptive field about the content of an image.
If you are doing work for an organization which uses the IPTC Title field to catalogue images, you should use whatever format they request. Although there is now a separate field for image filename, at one time there was not and the Title field served this function for many photographers and archival organizations. As such, it is probably best to assign a unique identifier to each image in the Title field. Something as simple as a date and image number code will do for most users.
Something that is the same for all files in a shoot that is descriptive would fit better in the Headline field, rather than in the Title field.
It's not uncommon to see some of the same IPTC information in various fields across images from the same shooting event, but the headlines are usually something that is appropriate for placing with the image in a publication. Thus, rather than filling the headline field of a portrait session with "John Smith Portrait Session" I'd probably go with something like "John Smith Portrait" or even "John Smith" and include the words "posed portrait" in the caption field, e.g. "Posed portrait of John Smith". You're also probably going to include the information identifying the human subject as "John Smith" in the Person Shown field.
I've often seen published photo galleries from an event covered by a photojournalist where the description/caption field for most of the images is the same. A few of the images might include additional information and might also eliminate some or all of the more generic caption used in the other images. For example, for images of a street fair the general description/caption for most of the images that show a wider view of a large crowd looking at classic cars parked in a downtown district and includes a nice blue sky might say something like, "Attendees enjoy the nice weather as they look at the classic cars on display at this month's Third Friday Downtown event." For a few images that feature closeups of individuals, the caption may read something like, "Sally Jones, age 7, enjoys the nice weather at the June Third Friday Downtown event." Or even, "Alice Davis,age 8, has her face painted downtown at June's Third Friday. Alice is the daughter of Bill and Jane Davis."