Chemical seal: Available only to full body armor that includes a helmet, the chemical seal is an airtight environmental control that takes a Complex Action to activate (if you’re already wearing the armor, natch). It provides complete protection against contact and inhalation vector chemicals, but can only be used for a total of an hour (the limit of the air supply) at a time.
Now here's a question... it outright says "full body armor". Which some would interpret to mean the specific item called "Full Body Armor" listed on pg 436. Avail 15, with a pricetag for the suit, a modifier for the helmet, and a modifier for the Chem Seal. No mention of using up Capacity, but it does say that the helmet still has 6 Capacity for vision enhancements and such.
But Helmets and Chem Seals are also listed on pg 438, with their own price tags, their own Capacity usage, etc. Which would mean that you can add Chem Seal to any full suit of armor, not just the "Full Body Armor" itself. Which is where I think OP is coming from. You could buy an Armored Clothing outfit, with a helmet, and add Chem Seal... it just uses up Capacity and costs whatever it costs. So does the required Capacity 6 for Chem Seal use up the available Capacity 6 in a store bought helmet? Or is there 6 more Capacity implied to helmets, reserved just for Chem Seal? Does the guy wearing Chem Sealed Armored Clothing and a motorcycle helmet, still have room left for vision enhancements?
Just wanted to point out that you can't always "do what it says", since we all know these books require a lot of cross referencing to get a clear picture. And sometimes there are contradictory rules that would confuse players instead. I agree, the easiest way to handle it is to say "Full Body Armor" says it can have vision mods, so the Armored Clothing guy can too. Or if they really meant for Chem Seal to only work for "Full Body Armor" then they should take it off the chart that also lists Nonconductive and Shock Frills.