The Merge command is only available on tree segments below a split directory, and only to those segments where only one of the two segments (Split directory parent or the selected node) has a fusk.
When you merge a segment with its split parent segment, the resulting segment is of the same type as the selected segment and has components mixed from both.
For example, the following tree contains two segments with these components:
Split Directory:
     
Pre-Text:    200907
     
Fusk:      None
     
Post-Text:  None
Selected File:
     
Pre-Text:    None
     
Fusk:      2 Digits from 1 to 21
     
Post-Text:  .jpg
Because only one of the segments has a fusk (selected file in this case), these two segments can be merged and the option will be available on the tree menu when the file is selected. All of the text “before” the fusk from both segments will become the Pre-Text for the new node and all of the text “After” the fusk from both segments will become the Post-Text.
Split Directory:
     
Pre-Text:    200907
     
Fusk:      2 Digits from 1 to 21
     
Post-Text:  .jpg