Projection is the rule the lens uses to fold a 3D world onto a flat chip. Rectilinear tries to keep straight lines straight. Fisheye families (equidistant, stereographic, orthographic, equisolid) trade that honesty for a wider view. A degree in one system is not a degree in the other.
In one sentence. If you compare FOV numbers across projections, you are comparing a ruler to a tape measure that was allowed to bend.
Rectilinear
This is the "normal camera" rule. Buildings do not belly. A robot can measure a bay if you calibrated. The price is that truly wide views get ugly at the edges, and past a point the lens simply cannot stay rectilinear without becoming huge or soft.
Fisheye, several kinds
Equidistant is common in machine vision: angle from the axis maps almost linearly to radius on the chip. Stereographic and the others are different maps. They all look "wide and bent." They do not unwrap with the same matrix.
The hallway argument
Someone wants "180 degrees" so the robot sees the crossing. A rectilinear 180° on a realistic M12 does not exist in the way they mean. A fisheye 180° does, and then their SLAM treats a curved door as a curved door. Either you live in the fisheye model, or you undistort and throw away the edges you paid for.
How to use the filter
Camemaker lenses have a Projection field. Filter it before you filter EFL. Then compute FOV inside that family. A 2.1 mm rectilinear and a 2.1 mm fisheye are not two strengths of the same lens. They are different instruments.
If the page is quiet on projection, do not assume rectilinear because the thread is M12. Plenty of M12 glass is fisheye and proud of it.