Codeception: What combinations of groups are possible?

Codeception allows to assign tests to one or multiple groups (see https://codeception.com/docs/07-AdvancedUsage.html#groups). These groups may be included or excluded when a testsuite is started (see https://codeception.com/docs/reference/Commands.html#Run) using --group [Groups of tests to be executed (multiple values allowed)] and/or --skip-group [Skip selected groups (multiple values allowed)].

I what way are these groups combined? AND or OR? How do group and skip-group interact with each other (has one precedence over the other?).