What exactly is a .ssf file? I am refering to the .ssf files used with Trimble GPS units. I am aware that it is some sort of post processing correction file, but I do not know more than that.
SSF is an acronym for Trimble's proprietary "Standard Storage Format" file.
Trimble Standard Storage Format (SSF) [...] describe coordinate geometry [...] information from Trimble TerraSync™ field software and GPS Pathfinder Office software.
A GPS file on your handheld data logger exists as several files (kind of like how a Shapefile needs multiple accessory files that all work together to make it work). Your Trimble GPS software will show it a a single data file, but if you looked at the directory in a file explorer, you'll see I think 6 or 8 files all with the same name, but with different file extensions. ALL of those files together make up the GPS datafile.
When you transfer the GPS datafile to your computer for post processing, all of these files are merged together into a single file, the .SSF.
So the .SSF file represents the totality of the raw unprocessed GPS data but in a single file that is compatible with the post processing software. It also aids in data management as you only have the single file to keep track of if you decide to save it somewhere else or email it to someone else to process.
If you transferred that .SSF file from your computer back to the GPS, it would be broken up again in to the multiple files.