how does gateway framework treat default system flag in customizing

Suppose we have maintain multiple backend system as “default” in customizing:

in the runtime, for a given service, ONLY exactly one system alias will be chosen.

The logic is, firstly all configured system alias will be retrieved together:

And if none of those alias is marked as default, exception will be raised;
If multiple have default maintained, only THE FIRST one will be returned ( line 29~31 )

This is the reason if I try to search one Opp with description QHD ( which is actually created in system QHD ) in gateway system which have configured both QHA and QHD as default but QHA is the first one, no result will be returned.

Just simply because gateway only issues a backend request to QHA, but not for QHD.