The importance of â, î, û (A¸ [ ¸ ]) can be understood from the shlöka :-

aum pūrnamadah, pūrnamidam, pūrnāt pūrnamudacyate
pūrnasya pūrnamādāya pūrnamevāvaśisyate. (Īś.Up.-1) & (Br.Up.- V.1.1)

This is the Shānti pāth at the beginning of Īśāvāsyam Upaniśad and it is also the first shlöka of the fifth adhyāy of the Brihadārānyaka Upaniśad. (This fifth chapter is the beginning of the third kānd or section, and is named khilakānd , the addendum). In both the Upaniśads this shlöka is importantly positioned. It means :-

Om That (Brahman) is Infinite, this (universe) too is infinite. The infinite (universe) emanates, originates from the Infinite (Brahman).

Assimilating the infinitude of the infinite (universe), the Infinite (Brahman) alone is left.

What is pūrna ? It is the root pūr, meaning full or complete and the suffix kta further stresses this meaning, making it more vehement or ‘that which is more than full’ i.e. Infinite. adah ( Ad: ) is ‘that’, a pronoun referring to something that is prökśa or ‘not known to the senses’. idam ( [dma\ ) is ‘this’, a pronoun referring to something that is pratyakśa or here and now. udañch ( ]dHca\ ) [20] is to arise from, to emanate. There are three layers of meaning here as follows :-

adah ( Ad: ) ; idam( [dma\ ) ; udañch ( ]dHca\ )

The syllables are two, three and four ; the â, î, û (A¸ [ ¸ ]) are each combining with the syllable d ( d ) to structure these layers and this is further explained in the next section comprising of three shlökas viz. Br.Up. – V.2.1 to 3. The latter explain the three d ( d ) as dam ( dma ), dān ( dana ), and dyā ( dyaa ) which are the qualities of the Gods, the Devas ; the people, manusya ; and the demons, the asuras respectively. This is asserted by the Goddess of speech, vāg devī as d – d – d (d – d – d ) the sound of the thunder in the clouds.