None. You can put MD on belt slot like you would do any other belt. However I don't like using mag harness too much (as I favour light more) so suit slot is the best place to put your MD. Of course if you are normal PFC without fancy role like mini-engi, or mini-medic, because they require 2 free hands to be effective.
how do i rank up? Like go to corporal or sgt?
Oinment pretty much only treats/prevents infection and slighty speeds up natural healing. Just like bandages that basically only stop bleeding. Medkit pouch and advanced medkit itself is what you really need to save space.
By the way, did anybody know that Advanced medkit can hold MRE? Just pick one up, throw that useless autoinjector and put MRE inside. With medkit pouch it saves a lot of space.
I can't figure out the whole "breaking into doors" thing.
Say that I'm planetside and the colony's power is out. I need to break into places and I can pry doors open with a crowbar. Why can some doors be pried open easily and others just give you the message "The airlock's motors resist your efforts to force it"? What affects this and can I circumvent this - and with which tools?
Leading on the first question, while hacking doors, what do the doorbolts exactly do and how do they affect the hacked doors in each of their states?
This means they are bolted. You would have to bring back the power to the door and then unbolt it with hacking.
Door bolts are basically like key lock in normal doors, they prevent opening the door by anyone. This serves many purposes on other servers (which I suggest playing first, then switching to CM13, otherwise you will find BYOND boring, unless you are benomain), on CM it means that benos won't be able to open such doors and they have to melt it. Marines can also shoot it with guns and it will also break. Bolting doors doesn't affect hacking in any way.
What's the difference between the Sentinel's neuro spit(25 plasma) and the neuro air splash(50 plasma)? I can't tell the difference in game and there's no detail about it on the wiki