"bak" generally stands for "backup". So I assume it's made by the tool you use to create that modpack and rar-file. I wouldn't mind.
hello, i have been working on creating a modpack for the famous technic launcher for minecraft when all of a sudden some random file appeared in the same folder of the .rar i was working with. the file in question is a 30373 file the full name is SSOM.bak13036.30373 (ignore the SSOM as it is the same name as the .rar. it takes up almost the same amount of space as the .rar. it also requires admin rights to interact with and even administrative rights wont let me delete it. i get the feeling that it is somehow related to my .rar in some way. help please.