Manual Postings: Fixed or Hidden
Subassignments 
The financial statement items you post manually sometimes require a breakdown by subassignments, depending on the item’s breakdown category. Journal entries posted with many items and subassignments can become very confusing.
For this reason, document type customizing provides an option to declare each subassignment as being fixed or hidden.
A fixed subassignment means that the subassignment must be unique per journal entry, and thus can only have one fixed value.
A fixed subassignment can either be permanent or temporary. When you fix a subassignment temporarily, you are able to deactivate the fixing when entering the document.

You may want to fix the subassignment transaction currency temporarily or permanently if you normally only enter documents in transaction currency. By making the transaction currency subassignment fixed, you can prevent the erroneous entry of two different transaction currencies.
In the manual posting transaction the system displays the fixed subassignments. The system shows the non-fixed characteristics for each line item.
You can hide subassignments that only apply to very few financial statement items.
The system only shows a hidden subassignment in posting when you enter an item that is broken down by that subassignment.