In writing, pcm means per calendar month — for each month of the calendar. You see it in British ads for rooms and flats: “£900 pcm” means you pay 900 pounds every month as rent. It is not a word you say out loud much; people write it to save space. Americans usually write per month or /mo instead. Calendar month is there to be exact: January, February, and so on, not a loose four-week stretch. Though actually, in a real ad everyone just hears “each month.” It has nothing to do with computers here, even though pcm is also a tech abbreviation in another world.

每月（英國房租廣告寫法；台灣會寫「月租」或「每月」）