According to Cambridge Dictionaries Online, PCM refers to the payment agreement: per calendar month. The agreed upon rent is paid each month regardless of the number of days within that month.
Red Residential's Guides to Renting explains this calculation more fully. Rent is often listed as a rate per week. Red Residential calculates the PCM rate by multiplying the weekly rent by 52 and dividing by 12. When a rental contract that identifies the rent to be paid per calendar month is drafted, the tenant owes the stated amount each month. Red Residential assures that tenants paying weekly or monthly pay the same amount over a six or 12-month contract.