Minecraft wiki entry for execute, associated text: Result
Unparseable if the argument is not specified correctly
In following conditions, terminates when this subcommand is not at the end of the subcommands chain; fails when at the end:
If <pos> is unloaded or out of the world.
If block at <pos> isn't a block entity.
If target fails to resolve to one valid entities (named players must be online)
If test doesn't pass.
When at the end of the subcommands chain, and the command is successful, success value is 1.
For if mode, result value is the number of matching data tags.
For unless mode, result value is 1.