I don't think there is a difference between "string" and "substring" in this context. I can see how it may be confusing to see these two functions presented side by side, and you're trying to figure out what the difference is, and the only difference you see is in that wording. I think the difference in wording likely comes from the documentation, and is a result of FIND being introduced in a much later version than INDEX.
If you look in the documentation for a function, there is often a section at the bottom which explains how a function differs from similar functions. For FIND, it says:
The FIND function and the INDEX function both search for substrings of characters in a character string. However, the INDEX function does not have the modifier nor the start-position arguments.
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