Clicking on a unit then clicking on a structure transfers the unit's armor type displayed onto the structure's armor type displayed on the UI (on click or hover-over). I still don’t know what armor type~~~~ structures really have.
For example, I click on a Tacmarine squad that has infantry armor, then I select a structure and look at the tooltip of it. It’ll say ’[X structure health] (Normal Armor)’. If I select the structure after having selected a heavy infantry unit, the tooltip of it will say ’[X structure health] (Heavy Armor)’. Confusing.