There are two distinctions between the judgement on Rosh Hashanah and the judgement of a person in the afterlife:

1) The soul is judged after death, and the body is judged on Rosh Hashanah.

2) After death it is impossible to repent (and this is one of the worst punishments, when a soul comes to the World of Truth and wants to repent but cannot). In the judgement of Rosh Hashanah, however, repentance is effective, and G‑d’s hand is extended to accept penitents.

Sefer Hama’amarim Kuntreisim vol 1, p. 71