In this mantra The Immortal and non-dissectible nature of Ultimate reality is expressed. Throug this Mantra the Immortal and non-dissectible nature of Ultimate reality his manifestation or form  i.e. Soul or Jiva is also proved as Immortal and non-dissectible, Ultimate reality

At the every pious occasion and holy rituals this mantra necessarily chanted. in Hindu Traditions.

Purna-Mantra - Meaning

ॐ Par-Brahman is Whole/complete/self-sufficient, He evolved from Wholeness/completeness that is why is called Ultimate whole or Ultimate Reality,
Becaulse by extracting complete/whole  from complete/whole reminder is always complete/whole

The Complete or mantra of Wholeness is testifying mantra of believing in (Sanatana or immortal/all time) Sanatana Indian Traditions