Bal primarily marks correction of a mistaken expression: the speaker initially says one thing and then introduces a replacement or adjustment.
«بل: لتدارك الغلط، بإقامة الثاني مقام الأول»
Bal corrects an earlier statement by setting the second in its place.
Its core function is neither the conjunction of waw nor the delay of thumma. The second expression is introduced to correct the earlier expression.
However, the legal effect differs according to whether the speaker is making a report (ikhbar) or a performative utterance (insha’) whose effect may already have occurred.