Base Unit of Measure:
A unit of measurement that can be determined by taking one measurement without having to combine any other measurement. Length, mass, and temperature are examples of base units.
Alternative Unit of Measure:
A unit of measurement that is determined by combining one or more measurements. In short, all units of measure other than the base unit of measure are referred to as alternative units of measure.
Read More About Unit of Measure (UOM) in SAP MM
The conversion factors for conversion from base unit of measure to alternative unit of measure can be found I table MARM.
Quantity in alternative unit of measure = (Quantity in bae unit of measure * MARM-UMREN ) / MARM-UMREZ
Read Here for SAP MM Forums.