From Bash's manual
6.4 Bash Conditional Expressions
Conditional expressions are used by the [[ compound command and the test and [ builtin commands.
Expressions may be unary or binary. Unary expressions are often used to examine the status of a file. There are string operators and numeric comparison operators as well. If the file argument to one of the primaries is of the form /dev/fd/N, then file descriptor N is checked. If the file argument to one of the primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2, respectively, is checked.
When used with [[, the ‘<’ and ‘>’ operators sort lexicographically using the current locale. The test command uses ASCII ordering.
Unless otherwise specified, primaries that operate on files follow symbolic links and operate on the target of the link, rather than the link itself.
What is the definition of a primary?
What is the difference between a primary and an operator or operation?