Artillery Units & Stockpiled

[Kool Kat]

Gents: 

When an artillery unit displays a "Stockpiled" message... what exactly does that mean? 

I understand that the unit has accumulated excess shells... so in game terms does that translate into the unit being able to fire multiple turns before it becomes "Unavailable?"

Also, what are the game conditions that lead up to an artillery unit becoming "Stockpiled?" For example, does the artillery unit need to be in close proximity to a supply source?