Multiple Instance Selection is a term that describes multiple selections.  In text editors or text tools, this is when the user has the ability to select more than one instance of text continuously.

It was actually one of main reasons some start their own editor projects. Some feel that "buffers" and "singular selections"  (as seen in Emacs, Vi, etc.)  are less useful in advanced text editing compared to MultipleSelections and TextReferencing. Life is too short to be memorizing what is in your buffer or the clipboard. 

Only a few editors use multiple instance selection.. and they call it a few different things, not always "multiple instance selection". 

Rectangular selection is a nice ''feature'', but multiple instance selection is a nice ''power''. You can even select multiple rectangles using multiple selections.