Commercial inquires are for anything you intend to mass distribute, profit off of, use to market, or any other commercial use. EG: Logos for your business, artwork for fliers, merchandise designs, vtuber models, character designs for commercial purposes, writing for a visual novel game, etc.
Personal inquiries are things that will not be sold, redistributed, or branded in any way. EG: A poster for your own home, an avatar for your social media (that will not be sold on merchandise), art of your character to show your friends and followers (but not sell or redistribute[fliers, website banner, etc] in any capacity), art of your dungeons and dragons character for a homebrew campaign with your friends, etc.