The dot is for tube artillery, not missile. Under the NATO symbology there's a separate symbol for "missile", and there's a peaked cap for "rocket" with an example of MLRS. There's also a similar arrow symbol for "mortar".
I'm not sure that BT differentiates between Tube v/s Missile.
But either way, my point is that Artillery regardless of type, has its own unit on that chart.
Its not part of the Tank Brigade.
And heavy assault regiments are noted for having extra formations of organic artillery.
Those are Mech regiments "Heavy-Assault" is the name of a Mech unit.
SLDF Tank regiments are comprised of a single type of tank.
Looking at the black line as Heavy makes me think a brigade like that would be compromised of 108 Each of 3 types of tank, say, Manticore, Puma, & Partisan for the AA regiment.
Or for a division assigned to urban deployment, maybe the Heavy Regiments would be made up of wheeled Demons & Magi infantry support tanks.
Or for an extra "kill mechs" sort of feel, Demolishers & Alacorns for the 2 regiments.
I don't actually recall any other tanks besides that Partisan that are fluffed as being AA focused.