The Library Maker isn't very polished, so there are still some "not useful" features left over in it.
Execution: Code is for custom library actions which need to execute more than a simple function, so they cannot simply select "Function". It was not intended for Code Actions, because Code actions are intended to only execute the code that the User enters, not the code that the Library enters. Despite this, Mark probably got confused when he made the official library, as he decided to mark Code actions as using the Execute: Code for it anyways. Either way, Code actions ignore the Execution type.
Interface: Code editor is more or less useless and is ultimately ignored. Originally, it was probably intended to indicate that the appearance of the action should be a code editor (as you'd expect). In the end, it was overridden by the special Action Kind: Code. Code actions still select Code editor as their Interface kind, but I doubt it would make a difference if they selected something else.
I hope this helps.
I am by no means an expert, as I haven't played with the different options very thoroughly, but I have done a fair bit recently for my upcoming LibMaker
, so I know a bit. If someone knows the system better than me, by all means, step forward and correct me ;-)