Clarifying Cover, Concealment, and Hiding

The rules around visibility, cover, concealment, hiding, and targeting can be difficult to parse and have some missing elements. Thus: a glossary, below, and some additional explanations around the terms used and steps necessary to sneak around.

Cover: You get behind an object. Partial cover is +2 to AC and Dex saves, half cover is +5, and full cover negates most attacks and renders you Unseen. 

Concealment: You're tough to make out. If you're partially concealed attacks on you have 25% miss chance, if you're totally concealed you're Unseen.

Sight: You can be observed, unseen, undetected, or unnoticed.

Observed: default. 

Unseen: attackers know your square, but cannot see you. 50% miss chance. 

Undetected: attackers are aware of your presence, but not your square. They have to guess at your location, then treat you as if you're Unseen. 

Unnoticed: attackers are unaware of your presence. If an attack is made against your square for whatever reason you benefit from 50% miss chance.

Lighting Conditions 

Bright Light: default. 

Dim Light: targets are partially concealed. 

Darkness: targets are totally concealed.

Hiding: you must have half cover, partial concealment, or an ability to hide. The creatures you're hiding from must be opposite the cover or etc. If you are observed you may break sight to become Unseen. If you are Unseen you may move and hide to become Undetected. You cannot become Unnoticed.

Seeking: If a creature is Unseen you can Seek to treat it as Observed but partially concealed or half-covered instead of Unseen. If it is undetected you can Seek an area 30 x 30 to move it from Undetected to Unseen (crit success to Observed but concealed/covered). If it it unnoticed you have no reason to check. If you make a check anyway (as a security sweep or etc.) pick a 30 x 30 area. Success moves them to Unseen, crit success to Observed.

Fog/smoke/foliage/etc. generates partial concealment, usually. Clutter/corridors/etc. is probably cover.