Yā ṭālibu marks an intended person; yā ṭāliban is an unspecified indefinite vocative. Their difference is in definiteness of address, not distance.
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Two Different Indefinite Vocatives
Vocative Particles: Five Forms, Uses and Parsing
Yā ṭālibu marks an intended person; yā ṭāliban is an unspecified indefinite vocative. Their difference is in definiteness of address, not distance.