Command Categories

Member Commands

As the name suggest these type of commands can be used in members.
Syntax:
"key": "#<command_name>:param(s)"
The result of the command is going to be replaced with the string.

Key Commands

As the name suggest these type of commands can be used in keys.
The word 'key' again comes from the JSON format, the equivalent in GameMaker would be member.
Syntax:
"#<command_name>:key": { ...params... }

Important

Its important to know that key commands are not a part of the resolved key.
In fact: "#some_command:some_name": { ... } is going to be resolved as
"some_name": { ... }

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Author's recommendation: next read #ref