tsofp.va contains the serial number for games used by Electronic Arts including The Sims 1, The Sims Online, The Sims 2, and a few console games for the PS2 and original Xbox, on the basis that the serial alters the hexadecimal numbers textually represented in this string:
Boxed copies of The Sims Online make modifications to the numbers within the first two "30"s of the string. The TSO trial download does not make use of a serial and so uses the string unaltered. This exact string can also be found in the RangerX.iff files in The Sims 1. If tsofp.va is invalid or missing from the game folder, the game will silently refuse to run.
In the installation files, tsofp.va does not reside in the cab files, but alongside them (under the same name, tsofp.va), so that the installer can read it directly and modify it with the serial key.
Games which use this string