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From the Model Mayhem FAQ:

What does TF... mean?
If you see the shorthand “TF” used, especially when browsing casting calls, it means that the work is a time trade for either prints, portfolios, or whatever the two participating parties agree to. The most common forms of TF are:

- TFP – Time for Pictures/Prints/Portfolio
- TFCD – Trade for CD of Images

There are other kinds of time trades out there - just ask if you want to know what you're trading your time for.