I'm planning to create a digital archive of family photos which will include data on family name, description of the scene in each photo, etc. I'd like the data to be stored with the photo and readable by other software. So I was thinking of storing it as EXIF data or something similar. But most software I've looked at allows me to create my own tags too.
So the questions are: should I store the data in existing EXIF/IPTC categories (like keyword or title); should I use IPTC or EXIF; and is it safe to create my own (safe for portability)? And if creating my own, does it matter if they're associated with the EXIF or IPTC data (or are user-created descriptors off somewhere on their own)?