Errata: via Games Workshop
A Ten-man unit with this special rule can break down into two five-man units, called combat squads.
You must decide which units are splitting into combat squads and which models go into each combat squad, immediately before deployment. A unit split into combat squads therefore is now two seperate units for all game purposes including calculating the total number of units in the army, and the number of units you can place in reserve. Then proceed with the deployment as normal. Note that two combat squads split from the same unit can embark in the same transport vehicle, providing its transport capacity allows.
This is downright interesting, and I am sure has tons of implications to strategy/tactics across the spectrum of the game (for marines).