جَاءَنِي زَيْدٌ بَلْ عَمْرٌو
Zayd came to me—rather, ʿAmr.
«Bal» signals redirection or iḍrāb from the first mention to the second. In a positive assertion, the rhetorical force is not the simple inclusion of both people as with «wa»; the second replaces or corrects the first focus, though grammatical accounts differ in some details.
The correct interpretation must account for whether the preceding statement is affirmative or negative and what misunderstanding the speaker is correcting.