Field help (BISQTY) 

Upon setting up an analytical nature (Common data/Accounting Tables/Analytical/Natures), whether there is work unit entry is defined, then a default value is attributed to it by specifying its default value (in reporting currency).
It is this unit that will appear during document entry. The quantity is automatically calculated by dividing the Ex-tax amount (converted into reporting currency) by the work unit value.
SEEINFO The quantity can be modified.