Minecraft wiki entry for execute, associated text: Result of condition subcommands
Unparseable if the argument is not specified correctly.
In some cases, for example, testing a block outside the world, both if and unless terminates or fails.
When not at the end of the subcommands chain, only if the condition tests pass is the subcommand following it executed; otherwise it terminates. If it is executed multiple times by multiple variables (e.g. executors, execution positions, execution rotations), it acts as a variable filter - Only the variable that matches the condition executes the next subcommand.
When at the end of the subcommands chain, it checks whether the condition is met and then outputs.