Forward observers (FOs) are generally scout units with company level attachments under the art tab and battalion attachments (typically heavier stuff) under the log tab. Note that the M981 and AMX-10 VOA are also forward observer units now. To select a barrage disable the other ammo types then use fire pos. Note that FOs each have a radio spotting range. If a single FO is in range to a target, any FO can hit that target with corrected fire, allowing for much heavier barrages. You can fire uncorrected too, but it's quite inaccurate so you'll need a lot of fire to have much effect (the AI has a limitation where their fire is always uncorrected for some reason and they like randomly hitting your spawn areas with DPICM).

Current aim times are 3 minutes for most tube and light rocket systems and 5 minutes for heavy MLRS. The Tochka (kind of a joke) takes even longer. These are still too short really.

USA/UK/W.Ger
Company level

• 155mm area barrage (6x guns)
  
• 155mm smoke screen
  
• 155mm DPICM

USA/UK
Battalion level

• 203mm area barrage
  
• 203mm point barrage
  
• M270 DPICM barrage

W.Ger
Battalion level

• 203mm point barrage
  
• M270 DPICM barrage
  
• LARS 110mm rocket barrage

France
Company level

• 155mm area barrage (6x guns)
  
• 155mm smoke screen
  
• 105mm area barrage

Battalion level

• 203mm area barrage
  
• 203mm point barrage
  
• M270 DPICM barrage

US/France airborne
company level

• 105mm area
  
• 105mm smoke
  
• 105mm point (probably unrealistic)

SOV/E.Ger
Company level

• 152mm area barrage (6x guns)
  
• 152mm smoke screen
  
• BM-21 rocket barrage (good for rapid suppression)

Battalion level

• 122mm area barrage using all 18 guns native to the battalion
  
• 203mm point barrage
  
• BM27/30 rocket barrage (to just destroy everything)

Soviet airborne
Company level

• 122mm area barrage (6x guns)
  
• 122mm smoke screen
  
• BM-21 rocket barrage (good for rapid suppression)

Battalion Level

• 203mm point
  
• BM27 rocket barrage
  
• Tochka U (for a laugh. But if you find a target that sits still long enough to hit it with a ballistic missile, please send a screenshot)