I am learning how to use ACLs and I mostly get it aside from one thing.
# setfacl -m user:jon:r-x /home/testuser1
# setfacl -m mask:r-x /home/testuser1
# setfacl -m user:jon:r-- /home/testuser1/project.txt
# setfacl -m mask:r-x /home/testuser1/project.txt
The above sets read/execute access to testuser1's home directory and read access to the project.txt file for the user jon. This, I understand. What's the "mask" entry all about? Can someone explain that?