Question about On Call artillery support

[Fred98]

When you give infantry an attack order, you have the option that On Call Support be “on” or “off”.

If you give artillery a fire order, again you have the option that On Call Support be “on” or “off”.

So it seems to me there are 4 variables:

Infantry: On Call Support “off” and Artillery: On Call Support “off”
Infantry: On Call Support “off” and Artillery: On Call Support “on”
Infantry: On Call Support “on” and Artillery: On Call Support “off”
Infantry: On Call Support “on” and Artillery: On Call Support “on”

Please explain the difference between the 4 variables.

[Arjuna]

Joe 98,
 
By attack, do you mean the checkbox for Arty Direct Support Only? If so, then this does not mean whether to provide on-call spt or not. Any indirect arty/mortar unit will normally do so as a matter of course. Checking this box means that any on-call spt units that are part of the attack group are restricted to only supporting units of the attack group - ie they cannot support units outside the attack group. This is on by default for an attack.

[Fred98]

So the answer is, to attach other artillery units to the group, before giving the attack order!
 
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[Arjuna]

If you want an arty unit to focus its support fire for the attack, then yes it's best to select it as part of the attacking force. You can easily do this by control clicking each of the arty units you want along with the attack boss and any other attachments you want.