to serve in a mustered company: to march out to war, or to take a turn of organized duty at the sanctuary
At its heart this verb pictures people gathered into an organized company and set to a shared task. It is the action behind the noun for a "host" or "army," and it always carries that sense of a body of people mustered and put to work together, never a lone individual acting on his own.
Two of its directions look opposite in English but are one idea in Hebrew. One is going out to war, close to the ordinary verb for doing battle (לחם) but picturing a mustered host on the march. The other is sanctuary service: the men who took their turn of duty at the tent of meeting, and the women who served at its entrance. Unlike the general verb for serving or laboring (עבד), this word frames that duty as an ordered company keeping a fixed term, which is why Scripture can describe the tent's workers with the same word it uses for troops.
A causative use adds a third angle: to muster others, to call up the people of the land for war. Here the subject does not serve but puts others into service.
Because no single English word spans both the battlefield and the sanctuary, translations reach for different words in each setting ("fight," "serve," "muster"), which quietly hides the one picture underneath: an ordered company answering a call to duty.