In Jainism, the next bhav (birth-form) and gati (realm) are determined at the moment of death. During life, the soul carries past karmas (prārabdha) and continues to accumulate new karmas (agāmī, sāñchita) through thoughts, words, and actions. But it is the karmas that ripen at the time of death that decide the next birth and the corresponding gati. Thus, the critical period is the final moments of life, when the accumulated karmas begin to mature into the next rebirth.

Notes:

• Digambar and Śvetāmbara traditions may describe nuances in how karmas ripen and how final-moments practice influences them, but the core principle remains: the next bhav/gati is fixed at or near the time of death based on the karmic balance.
• The goal in life is to reduce negative karma and cultivate purer karma through right conduct, right faith, and right knowledge, so that the next birth is more favorable (or even to attain kevala jñāna in due course).

If you’d like, I can summarize how Digambar and Śvetāmbara sources describe the moment of death and karmic ripening in more detail.