What are the matters in which it is permissible to seek divine guidance (istikharah) and those in which it is preferable to rely on reason?

It is not correct to divide matters into two categories concerning Istikhara (prayer for guidance); because Istikhara is legislated for all matters, minor and major, in addition to using one's intellect and studying the available options. If a person is hesitant about a matter and the right course of action is not clear to them, they entrust their affair to Allah and pray Salatul-Istikhara, which is a supplication to Allah, asking Him for success. Istikhara does not negate the intellect; rather, it complements it, balancing realism and spirituality. The Prophet, peace and blessings be upon him, used to teach the Companions Istikhara in all matters, as stated in the Hadith of Jabir, may Allah be pleased with him: "The Prophet, peace and blessings be upon him, used to teach us Istikhara in all matters, just as he taught us a Surah from the Qur'an."