“”In Jewish prayer, as in life, standing for something symbolizes a greater level of respect. Just as people stand to greet visiting dignitaries or the president, we stand during some prayers to indicate a greater level of respect and intention. The centerpiece of any Jewish prayer service is the Amidah, a prayer whose very name means “standing.” So, of course we stand during the standing prayer.

We also stand for Barekhu, Aleinu, Hallel, and as a general rule, one should stand any time that the Ark is open. It is customary to stand as the Torah is being paraded to and from the table where it is read, and when it is being held up and wrapped at the end of the Torah service. Mourners stand when they recite Kaddish. In most Orthodox and some Conservative communities the custom is for non-mourners to also stand for the other Kaddishes in the service (half Kaddish, full Kaddish, and rabbis’ Kaddish). Other Conservative communities and Reform communities tend to remain seated for Kaddishes. Some Reform communities stand for the Shema.

Now, what should you do when some people stand for a Kaddish? How do you decide if you’re one of the people who stands? Judaism values the custom of the community. So, if you don’t know what to do, look and see what the majority of the other congregants are doing. In the words of 9th century rabbi Natronia Gaon, “When they stand, stand; and when they sit, sit; and don’t appear different from the entire congregation.” If most are standing, the respectful thing to do is to stand. If most are sitting, take a seat. If it’s relatively evenly divided, do whichever you’d prefer, but be consistent — if you stand for one Kaddish, stand for them all.””

https://www.myjewishlearning.com/article/ask-the-expert-standing-in-shul/