A meta data value field or filter, is an element in a search, which works with meta data, eg. filtering out assets, whose keywords does not match any of the user's input:
A meta data element is created using the "Meta data field" tab:
The data, which such an element works with is the item_metafield.value table and column. This can be changed using the Draft - Field.
Everything, except the data, which a filter or value field does work on, can be edited using the The Details panel, ie the choice of meta data cannot be changed after creation