As Josh Carter described them, mobile conversion units resembled giant metal spiders. They possessed at least six legs, which connected to a large, circular "belly", a Cyber-conversion chamber. (AUDIO: Code Silver)
Mobile conversion units were used by the Cybermen of the Cyber-Mainframe as a highly fast method of performing Cyber-conversion. In particular, they were used as part of the dimension seeding protocols; the Cybermen would form a bridgehead from one universe to another and would secure a foothold, with mobile conversion units being used to slowly increase the Cybermen's numbers. However, this was described by a partially-converted Petronella Osgood as "slow", as the Cybermen did not convert many people when securing an initial foothold on a universe; indeed, one Cyber-Commander analysed that of the 93% of humans eliminated on the Fortress Island of N-Space, 64% of the humans had been deleted, and only 29% had been upgraded.
Despite this flaw, mobile conversion units were also useful as a form of hunter drone; Cyber-Controllers could broadcast updates through the Cyber-Mainframe to them containing medical data on a small number of individuals at one time, and the mobile conversion units could locate the priority targets and instantly convert them. However, the mobile conversion units' scanners could not detect their targets if they were hiding in an area protected from deep-range scans, although their logic circuits could be used to calculate whether or not the targets were still within scanning range.
In terms of physical weaknesses, mobile conversion units, like most Cybermen, could be destroyed with grenades. (AUDIO: Code Silver) They could also be temporarily shut down if their power connection to the Cyber-Mainframe was severed. Additionally, their power supply, like those of the other Cybermen, could be poisoned, such as if their power connection attempted to draw power from universes where the Cybermen's attempts at dimension seeding failed. (AUDIO: Master of Worlds)
Mobile conversion units could also be highly pre-judgemental when identifying the species of an unknown target. For example, one unit registered Josh Carter as human based on his dialect alone; this mistake would later prove vital to Josh's survival when another mobile conversion unit tried to upgrade him, (AUDIO: Code Silver) as Josh was able to use his Nestene enhancements to quietly take command of the conversion spider and discharge its power supply as an attack against the conversion unit itself, a Cyber-Controller, a partially-upgraded Osgood and several other Cybermen. (AUDIO: Master of Worlds)
When the Cybermen of the Cyber-Mainframe began attacking Fortress Island of the Earth of N-Space, the Cyber-Controller overseeing the invasion ordered for a number of mobile conversion units to be deployed against the UNIT soldiers on the fort, allowing the Cybermen to slowly increase their numbers whilst also securing a foothold on the planet. After Kate Stewart and Josh Carter destroyed a Cyberman, a mobile conversion unit confronted them and scanned Josh; concluding that he was a human after processing his dialect, the mobile unit attempted to convert Kate and Josh only to be destroyed by a grenade.
Shortly afterwards, the Cyber-Controller, as suggested by the partially-converted Osgood, uploaded an update to the remaining mobile conversion units, who registered Kate and Josh as their priority targets for upgrading. After failing to find either of the two, the mobile units concluded that Kate and Josh were hiding, as they were unable to leave the fort. Eventually, after receiving the pair's location via another update from the Cyber-Controller, two mobile conversion units assisted several Cybermen in attempting to convert Kate and Josh; although one unit was destroyed by another grenade, the second was able to secure Josh with several Mechanising Worms and tried to upgrade him. (AUDIO: Code Silver) However, Josh was able to use his Nestene enhancements to hijack the mobile unit. When the Master arrived, Josh "drove" the mobile unit into the fort's laboratory to seemingly convert Kate, only to then harness the mobile unit's power supply as an attack against the Cyber-Controller, Osgood, the mobile conversion unit and several Cybermen, allowing him to escape with Kate and an unconscious Master.
Sometime after waking from Josh's attack, Osgood, after eventually persuading the Cyber-Controller, modified a mobile conversion unit to upgrade the Master whilst retaining his symbiotic relationship with his TARDIS. When the Master returned to the fort with Kate and Sam Bishop, Osgood trapped the Master inside the modified mobile conversion unit; when the Cybermen's power connections were restored, the "Cyber-Master" emerged, who then upgraded the Cybermen's power connections to draw energy from all parallel Earths. All remaining mobile conversion units were destroyed shortly afterwards, as the boosted power connections included the parallel Earths that the Cybermen failed to commence full dimension seeding in, effectively poisoning their energy supply. (AUDIO: Master of Worlds)