Program Data in a POSIX environment?

Question

I'm a little confused on how/where program data should be stored in a Posix envronment, Debian Linux specifically. I understand that user specific data should be kept in $home, and looking at this question it looks like data common to all users should be in /var/lib/myProgram/ but how does myProgram get granted the access to read and write data there? I'm a little lost and I'm still a newcomer to linux, I'd appreciate any insights!