You wear the black, cumbersome, thickly-padded vacuum-proof suit of a grunt infantryman in the Black Lance Legion. Your Suit Durability (or SD) is like “hit points” for your suit. Your current space suit has 3 SD points, which is its starting amount as well as its maximum amount.
When your suit is damaged, its SD will drop. When you fix it (for example, with the Sticky Fix item) then your SD will rise, though never higher than its maximum amount. When your SD drops to zero or lower then your suit has been destroyed, and if you are in space when this happens then you will die automatically.
However, if your suit has sustained any SD damage at all, then that means your suit is punctured or torn in some way. If you are in the vacuum of space when this happens, and if the text prompts you to do so, then you will lose 1 Blood per SD damage. (That means that if your suit has sustained 2 SD damage while in space, and you cannot repair the damage, then the sucking cold of the void will drain you for 2 Blood at regular intervals!) This does not include the original section in which you lose SD, so if you apply Sticky Fix to your suit in the same section in which it was damaged, you will not lose any additional Blood. Note that every dose of Sticky Fix “heals” your suit 1 SD point.
If you are familiar with role playing games, then think of SD as the “hit points” of your protective armor. But since you are more often than not in an environment hostile to living things, then your armor must be perfectly sound in order to afford you constant, complete protection.