What are the Islamic sciences that are considered an individual obligation (fard al-ayn) upon every Muslim, and what are the limits of this obligation?

Seeking knowledge is of two types: an individual obligation (fard ayn), which is what a Muslim needs in their daily acts of worship and transactions, such as learning about creed, rulings of prayer, fasting, zakat, and Hajj, and rulings of buying, selling, and marriage. And a communal obligation (fard kifaya), which is specialization and deep immersion in advanced Shariah sciences like principles of jurisprudence (usul al-fiqh), grammar (nahw), and hadith, as well as worldly sciences like medicine, engineering, and military industries.