Can the substring widget be used to split a string?

I understand it can copy based on the character count, i.e. from 0-3 would copy the first four characters of the string.

In my case I'd want it to copy all of the substring text up until a % for example?

Sadly, there's no way to achieve what you're describing. You're correct with your understanding that the Substring Widget is used that way (e.g. set a start and end position).

From how you described it, you're looking for a way to split a string once it encounters a certain character (e.g. a %). This can't be done within JotForm but you can do this with the help of your developer.

Just have him grab the form's Full Source Code > deploy it on your page > then programmatically add a script to split a field using Javascript.

https://www.w3schools.com/jsref/jsref_split.asp