How to use multiple sdc files?

It is quite simple convenience, nothing technical.

You design a module named buddha_fft.vhd. You will use your full sdc file called buddha_fft.sdc that declares system clocks,multicycles,false paths but you don't care about io.

Once finished, you send your design to project integrator. He/She will read your sdc file and know any local constraints. Will convert them to project level names and add them to his/her sdc but may be more organized and put them in a separate sdc called buddha_fft.sdc after removing your project level constraints and editing node names.

I don't know about other applications of multiple sdc files. I may have several files to see which is better but will need to set project to one of them for a given build.