The defenders in a battle, started out with 18 CUs. After combat is resolved, attrion and battle casualties have been rolled for, there still is 13 CUs left in the stack.
Normally you are not allowed to move with more than 10 units.
May I retreat my stack of 13 CUs?
Or must I leave behind the 'excess' 3 CUs?
According to rule 15.1:
The 10 CU movement limit does not apply
during retreats; any size force (commanded by a general or not)
defeated in battle must be retreated.