"ALT" is an improper abbreviation for the Alternative Depreciation System (ADS) (see the link for a definition).
"MM" is the correct abbreviation for the Mid-Month Convention (see the link for a definition).
The mid-month convention states that all fixed asset acquisitions are assumed to have been purchased in the middle of the month for depreciation purposes. Thus, if a fixed asset was acquired on January 5th, the convention states that you bought it on January 15th; or, if you bought it on January 28, still assume that you bought it on January 15th. Doing so makes it easier to calculate a standard half-month of depreciation for that first month of ownership.
