A thing to remember here is that these posts are strictly meant for the particular planet position. Like, today we are looking at Jupiter in Sagittarius, so the description given here may change for you if your Jupiter is although in the sign of Sagittarius but with conjunction with or aspected by other planets.

So, lets see what Jupiter can do in Sagittarius without any conjunction or aspect from any other planet. 1st lets see what these 2 things represent --

Jupiter is the most benefic planet and it represents all the auspicious things like Knowledge, Wisdom, Law, Guru, Spirituality, Religion, Philosophy, Literature, Elderly People. For a girl, Jupiter also represents Husband.

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 next set of 2 and half nakshatra which are Mula, Purva-Ashadha and Uttara-Ashadha. Sagittarius' lord is Jupiter.

As we can see that Jupiter and Sagittarius are more or less the same thing and it should be as Sagittarius is ruled by Jupiter and it is Jupiter's Mool Trikona Sign, here it becomes the double impact of Jupiter in person's life. This person is all about knowledge, reading literature, different philosophies, about different religions and people of different races. This may even mean that a person may have a negative belief system, if he is raised in such type of society and family, and stands on it. Only thing he needs to take care is not to become fundamentalist of his own belief system. But normally, this person is about having a balanced outlook on different religions and beliefs. As Jupiter and Sagittarius both represent law of the land, this position can easily make someone a lawyer or judge. But this is excellent position for someone to become philosopher, teacher or preacher. A great position for Jupiter to be. 

Again, impact of Jupiter's effect in Sagittarius will be controlled by the various nakshatra Jupiter might be in. Like, if Jupiter is in Moola nakshatra, its lord Ketu's position will effect Jupiter. If Jupiter is in Purva-Ashadha nakshatra, then its lord Venus will impact Jupiter. If Jupiter is in Uttara-Ashadha nakshatra, then its lord Sun will impact Jupiter. 

Sign lord wise, Jupiter is in its Own and Mool Trikona Sign.