Free Worlds League: Static Defense Units - size/strength varies by planet due to local funding. Fits local conditions - armor, infantry, conventional air, naval, submarine, artillery
The way I understood it was that Static Defense Units were neither a militia nor a line unit, but rather a federal or provincial support structure containing non-line units--militia might be subsumed into this command, but were the branch itself was not militia. One of the personalities in the House Marik Sourcebook was in fact an officer in a static defense unit and a Marik-Brett. Static Defense Units represented a federal or provincial investment in having a prefabricated and ready defense structure for a planet that had charge over a planet's militia but also had charge over non-line regiments, and plausibly over line units in defense design, but not over the implementation of defense during an attack, which fell to the line, or militia commander (Whoever was in charge of the planetary defense). This fit in with the description of the Free Worlds League having an uncanny knack for defending against impossible odds as there was always and experienced adult who knew how to defend well sitting next to regulars and to the militia. This is why the units were listed as support units in the old Sourcebook.
EDIT: I will note that the Field Manual
strictly conflicts with the Sourcebook's explanation on this topic, which actually creates other problems. It is not a read between the lines issue and would make for a fair question for the writers. I guess it could be justified under Thomas's reforms but transforming them from support to infantry in a Successor State chock full of infantry is weird.
EDIT: There also could be more than one type of planetary defense unit within the jargon.