Most spas are equipped with one, two or three filter cartridges (depending on pump configuration). Vortex Spas™ (except for the Gemini) will have three filter cartridges.
The filter cartridge that is connected to your filtration pump will do most of the hard work in keeping your water clean. It is the left most filter if you are standing outside of the spa or the rightmost filter if you are inside the spa.
The filter face will have the words PZ written opposite the main/primary filter.
When you have an air lock, the main/primary filter hole requires flushing out with a hose to remove the air in the line.