Anyone else have their fallthrough macro fail?

[Sildenafil]

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/castsequence reset=0.3 1,lightning bolt
/castsequence reset=0.3 lava burst

The above macro works when spammed (but I mean, who doesn't lol) and no pauses in between my lightning bolt casts :P

what is the "1" in /castsequence reset=0.3 1, lightning bolt? Is it an item slot related ability specific to DKs and their weird weapon imbues, or is it an action bar button?