This is the slice collection editor, being used to inspect the in-game slice tree. You can use this to help debug scripts. If you're new to slices, it's probably a better idea to play around with slice collections in Custom rather than here.

You can also press CTRL+F8 while playing a game (exit this menu first!) to dump the slice tree to the file g_debug.txt. Alternatively, you can press F2 in this menu to export a slice collection to a file which you can load in Custom's slice collection editor and inspect later. 

Note that you can make some (but not all) changes to the slice tree here that you are not allowed to perform with scripts: if you do that, don't be surprised when everything goes haywire!

Press + to add a new slice to the current collection.

{{sliceedit_keys}}
