From Smart Merge
UnityYAMLMerge is shipped with a default fallback file (called mergespecfile.txt, also in the Tools folder) that specifies how it should proceed with unresolved conflicts or unknown files. This also allows you to use it as the main merge tool for version control systems (such as git) that don’t automatically select merge tools based on file extensions. The most common tools are already listed by default in mergespecfile.txt but you can edit this file to add new tools or change options.
Using TortoiseGitMerge as fallback
Edit C:\Program Files\Unity\Editor\Data\Tools\mergespecfile.txt
for fallback file and teach UnityYAMLMerge to use TortoiseGitMerge:
# TortoiseGitMerge
* use "%programs%\TortoiseGit\bin\TortoiseGitMerge.exe" /base:"%b" /mine:"%l" /theirs:"%r" /merged:"%d"
From Resoving Unity Scene Merge Conflicts with UnityYAMLMerge (Smart Merge) and TortoiseGit
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/rjtL7.png)