Stock photography print is a collection of a variety of generic photographs that photographers take with "no known usage" or "client", yet, but it may be sold (or not) at any time in the future. For instance, a photographer may hire a model and have her wear a business suit and take pictures of her pretending to be using a computer. He pays her usually a flat rate fee that covers the job and she signs away any financial rights to her 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 to perhaps a client who is looking for a more affordable option than booking their own model, photographer, etc., but the odds for many photographers are that they don't sell all of their stock (sometimes even very little).
If and when the photographer sells from their stock photography, they don't always know the intentions of how, where, and when it will be commercially used. The model will be aware that they are being booked for stock photography, so they should be aware of the pros and cons just in case no one informs them about why they should use caution in accepting this type of modeling booking during their career.
There is some controversy about whether or not agencies even accept "stock" photography modeling assignments for their models because some of the "cons" that outweigh the financial gain of the agency and model. It's really not that much of a gain "financially"; more or less $50, $75, $100, maybe $200. A model agency averages 15-20% of that rate, so to 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.
Models want to work and are always looking for photographic print opportunities especially if they are neither not very busy working as a model nor building their book with their print experiences from either fashion editorial or commercial work. Consideration should be taken when a model has a potentially promising career ahead of them because not only does the model have to sign a photographic usage release for the stock photos initially, but that model has no idea IF, WHEN, and HOW that print will be used when sold and that model gives up any residual gain of money, too, from that opportunity. The argument and hypothetical situation that agencies and models fear is that the stock photo gets used by a client that is a direct conflict of interest for a larger paying job or campaign in a model's future booking.
For example, the photographer sells the model's stock photo that they were paid $150 for to a local bank (that uses the image for their brochure ad and website's homepage). A few months later, that same model is selected by a large advertising agency for a big named national bank's campaign that is a huge commercial print opportunity including being on billboards that pays a huge sum of money. That becomes a direct conflict of interest that their face and image is associated with another bank, so kiss the big, national opportunity and money good-bye.
It's a gamble when accepting the stock photo assignment, but when a model wants experience, print work, and a "paying" job...the stock photo booking may be tempting especially when you need the money to reinvest into your career or to help pay your bills. A lot of photographers make money selling stock photography, but there are many, many stock prints that will never be sold. Let me repeat that point, there are many, many, stock prints that are never sold to anyone, anywhere, nor ever used again. That's part of the gamble for the photographer and model.
Some of the good points in favor of a model accepting a stock photography assignment may potentially outweigh the risk of the worst case scenario (regarding booking a similar client) ever even happening. There are some fine things that a model can take away from a stock assignment especially if they are new models who need commercial prints to help "market" themselves in their portfolios, on their comp cards, working with different photographers, etc.
Models increase the likelihood of getting future commercial bookings when clients see their commercial pictures. Keep in mind that not everyone can just look at a model without any pictures and visualize their potential as a model without seeing proof of how photogenic they are, how well they show the product, how well they pose, how well they fit into character of their product, etc. That is one of the big purposes of a model having a portfolio because clients, photographers, and agents use the model's portfolio as such an important reference.
Models can end up paying photographers for testing when the model is the one who needs certain types of pictures to market themselves, even in all parts of the country located near or far away from the primary fashion modeling areas and secondary modeling markets. Photographers always seem to be able to find people who want to be models and actors, so there's a lot of competition out there even for stock photography. Many modeling agencies will give the model the option and help them decide whether or not it may be right for them in their specific situation. The modeling agency generally knows the type of clients that they'll get and the types of models that they represent, so their guidance may help the model with their final decision. Remember, the modeling agency doesn't get a lot of money from such a smaller paying booking, so it may not be worth their time to set up and follow through with stock print jobs versus taking the risk of any future conflicts of interest between clients and models.