I have a similar design PVC drain, and you remove that fitting to be able to directly snake or cleanout the drain. Snaking throudh that port bypasses the P-trap and goes directly down the horizontal drain pipe. This is the type of drain. https://www.siouxchief.com/products/drainage/residential/floor-drains

Bruce

That makes sense. Thanks.

The last time I cleaned this out I used the shop vac. Which worked pretty well.