The searchhelp VMVA consists of a set of other searchhelps, including VMVAA "Sales document according to customer PO number".

In VMVAA, the field "Transaction group" TRVOG gets its value from the SAP parameter id VTV (see transaction SE11). The parameter VTV is set by the SAP application depending on the transaction from which the search help is called.

In  GuiXT there is also a way to set this parameter :

InputField ....  searchhelp="VMVA" shselname1="$VTV" shselvalue1="1"

i.e. selection parameters starting with '$' imply a "Set parameter ..." before GuiXT calls the searchhelp.