Most fields fill in from the post automatically, and anything you type applies to every post the schema covers. Neither helps when the value differs per post and lives somewhere else — a price in a custom field, a rating from ACF.

Set Where it applies first: the fields offered for mapping are the ones that exist on the post types the schema targets, so a schema aimed at Products offers product fields rather than every key on the site.

Every field then has a source selector above it. The options are:

Type a value — a fixed value, used on every page the schema applies to.
From the post (automatic) — the built-in behaviour: title, excerpt, featured image, author, dates.
From a custom field — choose from the custom fields found on the targeted post types. The list is read from the 300 most recent posts of those types — every public post type, on a schema that has no targets yet — so that opening the screen costs the same on a shop with ten posts and one with a million. A key used only on much older posts, or only on a post type that is not public, will not be in the list: pick Type a field name… and enter it, which also covers a field that exists but has no value stored yet.
From an ACF field — appears when Advanced Custom Fields is active. Reads through ACF, so return formats and options pages work as you expect.
From the author profile — any user meta key on the post's author.
From a category or tag field — term meta on a term attached to the post.

Choosing anything other than "Type a value" hides the text box, because the value now comes from elsewhere. Switching back to "Type a value" restores it.

If a mapped field is empty on a particular post, the schema falls back to the automatic value where one exists, and otherwise omits the property — better an absent property than an empty one.

Reading values from your own fields