What is 'tempo-sync mode'?

I am importing a few audio files. Some of them can be tiem stretched, some cant; I get an error that says "time stretch is not supported for clips in tempo-sync mode’. I did a search of the manual for ‘tempo-sync mode’ - there is nothing in the manual about this.

Any ideas?

Only thing I can think of is Musical Mode. Do they import? If so, does the pool reflect the correct starting tempo?