By default, the waveform that you design in the first tab is considered as "Single Read".
Second tab is considered for " Single Write". Similary third one is for "FIFO Read" and 4th one is "FIFO Write". If you are following this you don't have to select any waveform specifcially.
If you are performing "Single Read" then you need to perform a dummy read to register XGPIFSGLDATLX. This will automatically initiate the GPIF waveform for SIngle Read.
g_data = XGPIFSGLDATLX;
In case of FIFO Write,
GPIFTRIG = FIFO_EpNum; will trigger thr corresponding GPIF FIFO Write waveform.
But you can also use all tabs for different "FIFO Write" waveforms in case your application demands that. But then you need to select the waveform with the help of GPIFWFSELECT register.
Please let me know if my explanation is not clear.
Ok, it's all clear for me now, great thanks.