What is the bash `posix mode'?

What is the bash `posix mode'?

Although bash is an implementation of the POSIX.2 shell
specification, there are areas where the bash default behavior
differs from that spec. The bash `posix mode' changes the bash
behavior in these areas so that it obeys the spec more closely.

Posix mode is entered by starting bash with the --posix option or
executing `set -o posix' after bash is running.

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