FEMS (Form EleMent Selection) refers to the effectively acting local filter.
To calculate the result of a query, the elementary building blocks required for query generation are defined. A building block of this kind is generally a key figure with a filter. This is created either directly from a selection or indirectly as the operand of a calculated key figure. The filter is made up of all participating filters.
If you have created a calculated key figure Sales in North America for example, with the formula Sales in USA + Sales in Canada, and use this calculated key figure in a selection (structural element) containing a filter on the year 2015, two building blocks are required for the calculation of the result:
Sales in USA, 2015
Sales in Canada, 2015
This kind of effective local filter is known as a FEMS.
If multiple structures or nestings of calculated and restricted key figures are used, all of them go into the FEMS in the filter acting on the operand.
For more information, see SAP Note 2302681 .