$ man chmod
...
A numeric mode is from one to four octal digits (0-7),
derived by adding up the bits with values 4, 2, and 1.
Any omitted digits are assumed to be leading zeros. The
first digit selects the set user ID (4) and set group ID
(2) and save text image [`sticky'] (1) attributes. The
second digit selects permissions for the user who owns the
file: read (4), write (2), and execute (1); the third
selects permissions for other users in the file's group,
with the same values; and the fourth for other users not
in the file's group, with the same values.
...