Creating Scratch layer gives me a choice between Polygon and Multipolygon, so I started wondering how should I make use of them.

Reading a new book "Mastering QGIS" (through this answer by Joseph to What does buffer mean in QGIS Voronoi dialog?), authors' comment was "ESRI shapefiles store geospatial data in multi- type geometry, so using multi- type geometry is a good practice if you plan on converting between formats."

How should I choose one of them (Polygon or Multipolygon) when I create a new polygon layer? (I had confused multi-polygon with multipart-polygon, so I used to choose Polygon.)

One recent situation I have come to choose Polygon/Multipolygon was Changing parallel line length to split polygon in QGIS? in which I draw a dummy polygon as a background layer. Working mainly on shapefiles it seemed I was supposed to choose Multipolygon, although I needed only one polygon. In this case what I needed was simple polygon geometry without any attribute (Not sure this example make any distinction.)