Computers and their use have been portrayed very differently in various 40K products. Be that as it may, in THIS campaign computers will exist and be standard fare on every civilized world, much like they are employed on RW Earth today. Without computers a hi-tech society like that of the Imperium could not exists. That doesn't mean that the average citizen of the Imperium knows how computers work, only that they do! By the Throne, few people on ancient Earth know how they work; just take that ignorance and multiply it by a factor of ten and then apply a medieval-like superstition regarding all things technological to get an image of how computers fit into this campaign.

Almost every hi-tech appliance contains some form of computer processor, but purely mechanical low-tech devices usually does not. For example, an autopistol has an integrated processor that keeps track of ammunition expenditure, barrel wear, works the ammo feed system, clears most jams, manages sighting aids and recoil compensation and so on. No wonder perhaps, that the owner prays to the spirit in the machine before going into combat! On the other hand, a simple stub revolver is just a mechanical device; no computers are involved. But it probably doesn't hurt to use blessed oils when cleaning it.

Tech adepts can interface (using whatever interface technology they posses) with everything that has a processor onboard. Most processors will have some sort of security systems and encryption protocols, but smaller systems (like the aforementioned gun) are not very hard to break into. On the other hand, a smaller system is also very limited – a tech-priest could jam the guns electrical feed, turning the gun into a manually operated single-shot device, but not much more than that.

More advanced systems offer greater opportunities. A tech-priest that is able to access a Marine's power armor could do great damage (on the other hand the hallowed Astartes PA does not accept wire-less calls from strange tech-priests, so getting in might not be so easy). Larger computer networks also have additional security measures, including dedicated monitor-servitors and tech-priests responsible for the system's purity.