Vocative Particle

Vocative particle حَرْفُ النِّدَاءِ

Vocative particle is used to call someone e.g O boy, O girl, O Sidra etc

Example 1: 

O Ahmed
يا أحمَدُ
ya Ahmadu
Here Ahmadu took dammah.

Example 2:

O father of Ahmed
ya Aba Ahmed
Here Aba Ahmed is a possessive phrase, Aba (Father) is mudaf and Ahmed is mudaf ilaihi. So Aba took fatha.