what is .arduino15 directory ?

in ~ (home) - I see .arduino , .arduino15 and then I have the "Arduino" directory, with a few different versions and projects.

The "Arduino" is synced between a few computers using "resillio sync", I have not been aware of the .arduino15 used for compilation, and not synced it.

What kind of mess may I be in now ? - are some components of that directory updated on one system, not the others ?

in linux, that is .arduino15. the . at the front makes it hidden in linux

this is where arduino keeps its preferences file for versions of Arduino IDE > 1.5. they changed the format, so they changed the file name, so Arduino IDE > 1.5 will not get invalid info from an outdated file

In addition to preferences.txt, .arduino15 is also where any hardware packages you install via Tools > Board > Boards Manager are stored.

so if I syncronize only Arduino directory, but not .arduino15 or .arduino
... will I end up in some sort of hell where Arduino thinks a board is updated, but it is updated on only one computer ?
or will same update be offered for each computer ? (san't say to have seen this so far)

It will offer the update on each computer.

I see it now, (tested). it seems to me there is no significant disadvantage by doing it this way.
thank you,