So I have had the same problem. But I finally figured it out.

Any unit that launches a 'Missile' is categorized as a "SILO" unit, eg: Tactical missile launcher, cruisers, and of course mobile missile launcher, also the Strategic missile launcher falls under this category as well. So anything that is housed in a Silo effectively.

For anyone in the future trying to add specific types of units and or categories not commonly known, here's how:
(Example: adding Missile units to Arty target category)
1. Navigate to: https://faforever.github.io/spooky-db/#/
2. Select the unit(double click) and then open the link as marked below in Red.

3. Scroll down until you find the categories section and identify something unique in that list to specify that unit.

In this case, I found 2: "SILO" and "SNIPEMODE".
4. Navigate to your order.lua and add wherever you like, for me I wanted my Base artillery to prioritize enemy Missile launchers and Artillery & offensively when attacking also counterfire them.
Also added SNIPEMODE and Tech2 Bot for those Sniper units(Base defenses & Experimentals also have this designation) & that pesky Cybran Rocket bot respectively.
CODE:

        Arty = "{categories.ARTILLERY, categories.SILO, categories.SNIPEMODE, categories.TECH2 * categories.BOT}",

Arty now targets(in order of Priority):

• Anything Artillery
• Anything that can launch Missiles
• Anything that shoots from long range
• Finally the Rocket Bot

Yeah, my post was mainly just to help any future newcomers because I couldn't find anything showing a more troubleshooting approach to a unit that doesn't have an obvious category name and wasn't shown in the original post(old forum) or here.

I don't normally go around dredging up 9-month-old threads XD

So I have had the same problem. But I finally figured it out.

Any unit that launches a 'Missile' is categorized as a "SILO" unit, eg: Tactical missile launcher, cruisers, and of course mobile missile launcher, also the Strategic missile launcher falls under this category as well. So anything that is housed in a Silo effectively.

For anyone in the future trying to add specific types of units and or categories not commonly known, here's how:
(Example: adding Missile units to Arty target category)
1. Navigate to: https://faforever.github.io/spooky-db/#/
2. Select the unit(double click) and then open the link as marked below in Red.

3. Scroll down until you find the categories section and identify something unique in that list to specify that unit.

In this case, I found 2: "SILO" and "SNIPEMODE".
4. Navigate to your order.lua and add wherever you like, for me I wanted my Base artillery to prioritize enemy Missile launchers and Artillery & offensively when attacking also counterfire them.
Also added SNIPEMODE and Tech2 Bot for those Sniper units(Base defenses & Experimentals also have this designation) & that pesky Cybran Rocket bot respectively.
CODE:

        Arty = "{categories.ARTILLERY, categories.SILO, categories.SNIPEMODE, categories.TECH2 * categories.BOT}",

Arty now targets(in order of Priority):

• Anything Artillery
• Anything that can launch Missiles
• Anything that shoots from long range
• Finally the Rocket Bot