Vyapti
Which part of the day a tithi must cover for the observance to fall there.
The rule that decides which civil day a festival lands on. Different festivals own different kalas: Maha Shivaratri takes nishita (midnight), Rama Navami madhyahna (noon), Diwali's Lakshmi Puja pradosha (after sunset), shraddha aparahna (afternoon), Ekadashi arunodaya (before dawn). A calendar that uses the sunrise tithi for everything will get most of the big festivals wrong at least sometimes.