How do I figure out what width the sector is 90 or 120 it doesnt say anywhere ?
3 dB Beamwidth - 90 degree,
6 dB Beamwidth - 120 degree.
You can find more information in datasheet:
So it does both ? How does this work could you explain more ?
Yep, it does both.
You should expect nominal gain at the very front of the antenna (18dBi), -3dB at +45°/-45° azimut from the front (15dBi) and -6dB at +60°/-60° (12dBi).