Seth's comment is correct, it uses the OS default.
But a more complete answer is you can also customize the default behavior for non-default editors through Tools > Options > Custom Actions. In theory at least, because with v188.8.131.52 it doesn't work too well because my editor remains in the background. But that's better than nothing, if you can't change the system default (can be inconvenient for HTML, for example). In my experience you have to restart SourceTree for this to take effect, by the way (the program doesn't inform you of that, which made me waste time on pointless "troubleshooting").
Supported Platforms macOS Windows To make using Sourcetree as simple yet powerful as possible we embed (bundle) dependencies such as Git, Git LFS, and Mercurial. We strive to keep these...
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