Product photography is a collection of various generic photos that the photographer take with "no known usage" or "client", yet, but it may be sold (or not) at any time in the future. For example, a photographer can hire product photography models and have them wear a business suit and take a picture of them pretending to use computers.
He pays them usually a flat rate fee that covers the job and they sign away any financial rights to their image in that photograph. The photographer then has those pictures obtained from that photo-shoot and adds them to his "stock pile" of photos.
He may sell that stock photograph for possible clients who are looking for a more affordable option than booking their own models, photographers, etc., but the possibility for many photographers is that they do not sell all of their shares (sometimes even very little).
If and when the photographer sells from their stock photography, they do not always know the intentions of how, where, and when to use it commercially. The models will be aware that they are being booked for stock photography, so they must be aware of the pros and cons just in case there is nothing to inform them about why they should be careful in accepting modelling reservations during their career.
There is some controversy about whether or not agencies even accept "stock" photography modelling assignments for their models because some of the "cons" that outweigh the financial gain of the agency and model. Some agencies that are very busy and aware of some potential conflicts, they may make the decision not to choose nor handle those bookings for their models.