Unit of measure UOM in Microsoft Dynamics 365 Finance and Operations

Unit of measure UOM in Microsoft Dynamics 365 Finance and Operations

Base unit
base for a particular class (weight, volume) and lesser conversions needed between the same class
If this check box is selected, the unit of measure is defined as the base unit within the unit’s unit class. The base unit helps streamline the conversion setup for a unit of measure. When a base unit is defined within a unit class, the number of conversion rules between units of measure in the class can be limited.
e,g, you can convert from one unit to other unit without a conversion rule between these two units.
 

System unit
D365FO uses when no UOM is specified when only a qty field is present, system unit is used to interpret the qty
e.g. You may assign base unit (within that class) and system unit (for a class type), the same, for easier understanding.

 
Fixed unit

(Length, mass, volume) (prior versions of AX, used in upgrade, integrations)

fixed unit association is used to identify a specific unit of measure. By using this association, specific units of measure can be identified by the business logic in Microsoft Dynamics AX and conversions between the units of measure can be completed.
E.g. You can name a unit anything / gm / grm for GRAMS which is the fixed unit known by outside world.