«جاءني زيد أو عمرو»
Zayd or ʿAmr came to me.
In this context aw (أو) presents an alternative without selecting a definite one. The uncertainty may reflect doubt about the event; elsewhere aw can serve other conventional functions, such as offering a choice. Context decides.
If both people certainly came and the speaker intends to report both, wāw would be appropriate. Bal instead shifts a previous attribution, while aw opens or leaves unresolved alternatives.
The reader should decide whether the speaker is reporting joint participation, a correction, or an alternative; the same syntactic position can support markedly different meanings.