I guess I can kinda answer my question. Looks like its an identifier from the bundle's info.plist
file.
...
<string>Sketch</string>
<key>CFBundleDocumentTypes</key>
<array>
<dict>
<key>CFBundleTypeIconFile</key>
<string>doc.icns</string>
<key>CFBundleTypeName</key>
<string>com.bohemiancoding.sketch.drawing</string>
<key>CFBundleTypeRole</key>
<string>Viewer</string>
<key>LSHandlerRank</key>
<string>Owner</string>
<key>LSItemContentTypes</key>
<array>
<string>com.bohemiancoding.sketch.drawing</string>
</array>
<key>LSTypeIsPackage</key>
<true/>
<key>NSDocumentClass</key>
<string>MSDocument</string>
</dict>
<dict>
<key>CFBundleTypeIconFile</key>
<string>doc.icns</string>
<key>CFBundleTypeName</key>
...
Some other types are
com.bohemiancoding.sketch.drawing.single
com.bohemiancoding.sketch.plugin
com.adobe.pdf
public.webp
com.adobe.encapsulated-postscript
public.image
com.bohemiancoding.sketch.plugin.bundle
com.bohemiancoding.sketch.clouddrawing.single