Salam,
Imami Shia claim `Ali had a distinct rank and position that no one else held, but did he clarify this in his official letters?
We find in Nahj-ul-Balagha, under the section on `Ali's letters, on page 582 in a letter he sent to the Koufans as he left for Basarah:
مِنْ عَبْدِ اللهِ عَلِيّ أَمِيرِالْمُؤْمِنِينَ إلَى أَهْلِ الْكُوفَةِ، جَبْهَةِ الاَْنْصَارِ وَسَنَامِ الْعَرَبِ أَمَّا بَعْدُ
"From the slave of Allah `Ali, commander of the faithful, to the people of Kufah who are foremost among the supporters and chiefs of the Arabs."
And in another letter he wrote regarding his Will:
هذَا مَا أَمَرَ بِهِ عَبْدُ اللهِ عَلِيُّ بْنُ أَبِي طَالِب أمِيرُالْمُؤْمِنِينَ فِي مَالِهِ، ابْتِغَاءَ وَجْهِ اللهِ، لِيُولِجَني بِهِ الْجَنَّةَ، وَيُعْطِيَني الاَْمَنَةَ
"This is what was ordered by the slave of Allah `Ali ibn abi Talib, commander of the faithful, regarding his property, seeking Allah's face."
On the other hand we see the leaders before him, such as `Umar ibn al-Khattab write their letters identically, like in al-Ahkam al-Sultaniyyah, the first volume, on page 122:
من عبدالله عمر بن الخطاب أمير المؤمنين إلى عبد الله بن قيس، سلام عليك، أما بعد
"From the slave of Allah `Umar ibn al-Khattab, commander of the faithful, to `Abdullah ibn Qays, peace be upon you."
And the leader after him, also all of them wrote the exact same words, such as abu Ja`far al-Mansour, in al-Kamil-fil-Lughah, third volume 1490-1491:
من عبد الله عبد الله أمير المؤمنين، إلى محمد بن عبد الله، أما بعد
"From the slave of Allah `Abdullah, commander of the faithful, to Muhammad bin `Abdullah."
In other words all of them except Abu Bakr al-Siddeeq wrote the exact same expressions, this is because Abu Bakr had a distinct station of being the actual successor, so he would write "Khalifatu Rasul-Allah" and the title of "Commander of the faithful" was not used in his time.
If `Ali had a distinct position and elevated status, why didn't he use it to identify himself? For instance Shia claim he's "The infallible leader" or they say "Wasi Rasul-Allah", why did he not use these titles to show his special position?
So ponder.