Including the year and month on a project file means it can stand independent of it's directory for easy distinction in search, meaning you can find relevant files in a way that is uncomplicated and easy to understand.
[Edit] As dkarl mentioned more elegantly: this system helps map a project in time.
YYMM on project files is shorter and less ambiguous than using a project name for the same function.
I'd imagine the sub-directory underscore is also used to aid searching. You can easily visually identify (or filter) sub-directories.
> The initial underscore helps by (a) keeping those directories sorted [separately] and (b) providing a visual clue that they are 'special'. [0]
[Edit] As dkarl mentioned more elegantly: this system helps map a project in time.
YYMM on project files is shorter and less ambiguous than using a project name for the same function.
I'd imagine the sub-directory underscore is also used to aid searching. You can easily visually identify (or filter) sub-directories.
> The initial underscore helps by (a) keeping those directories sorted [separately] and (b) providing a visual clue that they are 'special'. [0]
0. https://www.sitepoint.com/community/t/why-do-some-folders-ha...