Value must start with either
"owned", (only counting units owned by the attached player)
"allied", (which counts units owned by the attached players allies also)
or "total", (which counts enemy units too)
then after that is a colon delimited list of units or the word "all"
and then a count equal to the maximum number of units allowed in a territory.
<option name="placementLimit" value="owned:Battleship:Battlecruiser:Cruiser:Destroyer:Submarine:Transport" count="4"/>
It only affects placement.