Let’s look at important things here –
Mercury - Mercury represents communication skills, marketing, calculative ability, younger siblings, hobbies, skills with hands, intelligence, quick decision making, logical thinking, youthfulness etc.
Uttara Ashadha Nakshatra - It is a nakshatra of career driven people. People who look for authority in life and are highly ambitious. Please check this link to get more information on Uttara Ashadha Nakshatra - https://www.astrosaxena.com/uanaksh.
Sagittarius - As Uttara Ashadha is part of Sagittarius sign, Sagittarius and things represented by Sagittarius are important here. Sagittarius is the 9th sign of Zodiac Belt, hence it represents many things related with the 9th house of horoscope like Religion, Knowledge, Wisdom, Higher Learnings, Gurus, Teachings from your Father etc. Besides it also represents Coaching, Universities, Professors and Preachers, Higher Wisdom, Written Law of the Land etc. Sagittarius has 2 and half nakshatra which are Mula, Purva-Ashadha and Uttara-Ashadha. Sagittarius' lord is Jupiter.
Capricorn - As Uttara Ashadha is also part of Capricorn sign, Capricorn and things represented by Capricorn are also important here. Capricorn is the 10th sign of zodiac belt, hence it represents many things related with 10th house of horoscope like Law and Order, Government, Authority, Organisation, Discipline etc. Besides it also represents World outside your home and people, Fear and Anxiety etc. Capricorn is consisted of next 2 and half Nakshatra, i.e. Uttar-Ashadha, Shravana and Dhanishtha. Capricorn is ruled by Saturn and that's why it represents Fear and Anxiety.
Sun - As Sun rules Uttara Ashadha, Sun's position and dignity is important to know about the overall functioning of any planet in Uttara Ashadha.
Jupiter - As Jupiter rules Sagittarius, their position and dignity is important to know about the overall functioning of any planet in Uttara Ashadha/Sagittarius.
Saturn - As Saturn rules Capricorn, their position and dignity is important to know about the overall functioning of any planet in Uttara Ashadha/Capricorn
Houses Involved – Also, we can’t afford to forget the houses involved as a planet can be ruling different houses and sitting in different house for every person as per ascendant and planet’s position. To know overall affect, we have to consider the houses involved as those areas of life are being impacted.
Interpretation of Mercury in Uttara Ashadha Nakshatra – Here Mercury is in friendly nakshatra of Sun and in Sagittarius section it is in enemy sign ruled by Jupiter but in Capricorn section, it is in friendly sign ruled by Saturn. So, we can say that Mercury is far stronger in Capricorn section of Uttara Ashadha. As it is nakshatra of victories and achievements, it shows that Mercury here will make a person look for achievements in life through his business and communication. A person will take up his higher education received through Moola and Purva Ashadha Nakshatra and utilize the same in getting some recognition in world. This trait can be more prominent in Capricorn section as it is sign of authority and recognition. In Sagittarius section, person can still be inclined towards simply sharing the knowledge with others without much desire for getting the recognition or that desire can be hidden underneath the person, repressed and curbed. But when Mercury is in Capricorn, person will actively look for recognition through his business and communication.
As it is ruled by Sun which represents the ego, in relationship matters this position can bring ego clashes between partners and they would have to let go of their ego to sustain relation. Again, either you can keep your ego or you can keep your relationship.
Role of Sun – Sun placement is important to guide Mercury in right direction as it is lord of Uttara Ashadha
Role of Jupiter - Also, Jupiter placement is important to guide Mercury in right direction, being Sagittarius lord.
Role of Saturn - Also, Saturn placement is important to guide Mercury in right direction, being Capricorn lord.
Other Factors – And of course, we need to consider conjunction, aspects, house lordship, house position and dashas etc to know the exact results of Mercury in Uttara Ashadha. So, we always need to take a holistic view of chart rather than just 1 planetary position.
Conclusion – So, this is how I see Mercury in Uttara Ashadha can work in a chart.
Hope this helps. Please post any comment or query you may have.