Each file has an archive bit. This simple scheme sounds good in theory. In practice, it doesn't work very well. The problem is that the backup software has to trust that the bit is set correctly. It often isn't.
Various programs flip bits.
For example, moving a file doesn't reset the bit. Worse, the backup program itself may be the problem. Imagine creating a set of tapes and performing an incremental backup to it. All the archive bits are set. The first backup set the archive bits.