signal for (someone or something)
1. To use some kind of indicator, especially a hand gesture, to signify that one wants or needs someone to come over to one. I signaled for Tom to come help us with the machine. Will you signal for the flight attendant? I think there's something wrong with the seat.
2. To use some kind of indicator to signify something that should or needs to be done. We built a massive bonfire on the beach of the deserted island to signal for help from any ships that might pass by. Janet signaled for another round of drinks to be brought to our table.
signal for someone
to make a sign for someone to come. I signaled for the waiter and got the check. Ted signaled for the parking lot attendant.
signal for something
to make a sign that something should be done. I caught the waiter's eye and signaled for the check. The director signaled for applause.