Would like to give a brief explanation in how modifiers work in a macro and options you got.
/cast [mod] Spell X; - This one works with any of the 3 keys
/cast [mod:shift] Spell X;
/cast [mod:ctrl] Spell X;
/cast [mod:alt] Spell X;
/cast [mod:shift/alt] Spell X;
/cast [mod:ctrl/alt] Spell X;
This modifiers serve the purpose of adding extra utility spell by holding 1 of the 3 keys, could be "Alt", "Ctrl" or "Shift." If you hold the key, it would light up with the spell icon you input after the modifier. Remember this modifier goes before the actual spell you would like to cast for spamming.
/cast [mod] Rend; Heroic Strike
This macro would respond with no modifiers to a single ability which is Heroic Strike, if hold a modifier key like mentioned before it will try to use Rend ability.