Cisco Firepower 4100

You cannot add a subinterface to a physical interface that is currently allocated to a logical device. If other
subinterfaces of the parent are allocated, you can add a new subinterface as long as the parent interface itself
is not allocated.
Subinterfaces are supported on data or data-sharing type interfaces only.
Example
The following example creates 3 subinterfaces on Ethernet 1/1, and sets them to be data-sharing
interfaces.
Cisco Firepower 4100/9300 FXOS Command Reference
77
A – R Commands
create subinterface