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 end of the subcommands chain; fails when at the end:
If <start>, <end>, or <destination> is unloaded or out of the world.
If the volume of the source region is greater than 32768 (the equivalent of 8 chunk sections)‌
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 blocks (the total number of blocks in all mode, or the number of source region non-air blocks in masked mode).
For unless mode, result value is 1.