If there's only one function in the tag:

• If there's no /return command, the "/function #tag" is void, having no success value.
• Otherwise, the success of "/function #tag" is whether the /return command in the function returns a failure or success.

If there're multiple functions in the tag:

• If there's no /return command in any function, the "/function #tag" is void, having no success value.
• Otherwise, the success of "/function #tag" is always true.

How to reproduce:

1. Download and enable the attached data pack. In which, there're two functions and three tags. The two functions (a, b) are exactly the same, and both return a failure with a /return command. The three tags include function a, b, a&b, respectively.
2. Run /execute store success storage name path int 1 run function #a
3. Run /data get storage name path
4. Found that it's 0. Because the /return returns a failure.
5. Run /execute store success storage name path int 1 run function #b
6. Run /data get storage name path
7. Found that it's 0. Because the /return returns a failure.
8. Run /execute store success storage name path int 1 run function #anb
9. Run /data get storage name path
10. Found that it's 1. Though both /return commands return a failure.