Significance of *Ukaaram (உ)*
It is customary in sanskrit to start writing with श्रो: (Sri:)
श्रो: represent mahalakshmi or piraati. In the pranavam Om (அ+உ+ம), the ukaaram, உ represent the piraati sambandham between Akaaram "அ" the paramaathma and makaaram "ம" the jeevathma.
Hence the reason behind using श्रो: before the start is that piraati removes all kinds of hurdles and hindrance and ensures prosperity and fullfillness in form of Sri Lakshmi Kataaksham.
The same custom would have prevailed in tamil also and became common to use the equivalent of श्रो: in tamil, namely the " உ " to start writing, be it a grocery list, letter or an invitation.
In many literary works the authors use either உ as stand alone or the combo of அ+உ+ம as the first letters to the words used in their first stanza.
Thiruvalluvar used all three in his first kural A(அ)kara...mu(ம+உ) dhala ezhuthellaamu
Kambar starts Kamba Raamayanam with U(உ)lagam yaavayum thaamula vaakalum.
Swamy Nammazhvar starts thiruvaaimozhi U(உ)yarvara uyarnalam udayavan yavanavan.
Thirupaanazhwar uses அ+உ+ம in his first three paasurams i.e. a(அ)malanaadhipiran; u(உ)yarndha and m(ம)andhipaai..
In olden days when palm leaves are used, the letter உ was first written to ensure the suitability of particular palm leaves to further write upon.
These show the use of ukaaram to highlight piraati or aacharya sambandham to dedicate their works at the feet of the perumal.
Thayaar only shows the feet of perumal to a jeevathma. But who will show the feet of Thayaar to a jeevathma? Its the aacharyan.
Avvaanavarku Mavvaanavar adimai enru Uvvaanavar uraithaar (அவ்வானவற்கு மவ்வானவர் அடிமை என்று உவ்வானவர்உரைத்தார் " ).
Hence for those like aazhwaars who are in prathama paruva nishtai, ukaaram is piraati. Those like us in charama paruva nishtai, ukaaram is always aacharyan.
Aacharyan thiruvadigalae charanam.