The output parameters of a called POU can be directly assigned upon being called in the text language ST.
Example:
Output parameter out1 of afbinst is assigned variable a.
ST:
afbinst(in1:=1, out1=>a);
If the POU is inserted via input assistant (<F2>) with option 'With arguments' in the implementation window of a ST POU, it will automatically be displayed with all parameters in this syntax. However you are not forced to assign these parameters.