Fixing or Hiding Subassignments 
The FS items which you post to manually, require a breakdown by subassignment according to breakdown category. When you enter documents with lots of FS items and lots of subassignments, then the document entry can be very unclear. Therefore you have the possibility in the Customizing of the document type for each subassignment, to fix or hide them.
Fixing means that the subassignment must clearly be in the document, so it is only allowed to have a fixed value. You can fix a subassignment permanently or temporarily. When you only fix a subassignment temporarily in Customizing, then you can deactivate the fixing when entering the line item.
Example
For example you could fix the subassignment transaction currency temporarily or permanently, when you normally only enter documents in a transaction currency. Through the fixing you make sure that you do not enter two different transaction currencies by mistake.
In the manual posting transaction the system shows the fixed subassignments on the tab page Clear Subassignments . The system displays the unfixed characteristics per document line item.
You can hide subassignments which only appear for a few FS items.
The system then displays these subassignments again, when you enter a FS item in the document line items, whose breakdown category is set up to allow the subassignment(s).