a vtol rearming pad
usually a player can have up to 50 of these. game settings allow limit to be set between 0 (none) and 50 (default). some mods exist that allow more.
vtols will continue attacking their last target (if it still exists) or their last patrol/circle after rearming - see http://forums.wz2100.net/viewtopic.php?f=10&t=8826&start=90#p95168