{Improvement}
{S_Name}Starport{/S_Name}
{S_InternalName}BasicStarport{/S_InternalName}
{!-- Note: This internal name is also used by code, so if you change it here, you'll have to change the code too. --}
{S_Description}Your basic starport. You need this to launch constructed starships.{/S_Description}
{S_Type}Normal{/S_Type}
{Cost}20{/Cost}
{Maintenance}1{/Maintenance}
{S_IconName}BasicStarport.png{/S_IconName}
{S_QueryGraphicName}BasicStarport.png{/S_QueryGraphicName}
{PlacementLimit}1{/PlacementLimit}
{/Improvement}

Well, since it states that the internal name is used by code, I suppose that's the one your looking for. Didn't try it out for myself, but I don't think it's something else.

You can use either BasicStarport or UpgradedStarport for S_InternalName to treat the building as a starport. I use UpgradedStarport for my modded starports because if you use BasicStarport and you already have one, you don't see the upgrades .

There's a pic for it in the \Gfx\Improvements directory...that's where I got the idea to use the S_InternalName.

Your guess is as good as mine