This case will be called when an assignment is encountered. The case-function (i.e. the function resulting from this case-statement) is called with the tag of the left-hand-side of the :=-expression (i.e. the invocation). Based on the tag of the invocation, we know whether we are assigning a variable (i.e. v:=...), a function (i.e. v(...):=... or v@...:=...) or a table (i.e. v[...]:=...).

Click on the functions to see more specifically how these possibilities are handled.