Profile feed or a community
Reddit is not one destination. The same caption can go on your profile or into a community, and those are different posts.
The community field
Leave it empty and QuickPost submits to your profile feed. Type php or r/php and the post goes to that community. A pasted community URL is reduced to the name.
Title, body, flair
The title is required and stops at 300 characters. The caption is the body. If you looked up the community in the cabinet, its flairs are listed and you can paste the id.
What you can look up first
Search and your subscriptions show member count, whether images and video are allowed, the rules, and the link flairs. That page is there so a later automation can pick a community without guessing.