I have created a custom post type and three custom taxonomies. I am no going to create a new field group.
I have several text fields etc, but I am also thinking to use three multiple choice fields for the three custom taxonomies. Should I choose Taxonomy field och Taxonomy Advanced field? Does my choice have anything to do with my custom table?
Where are these fields data saved, if I choose the first or the second type? Do any of these two fields save the data in the custom table? In your documenmtation you say about Taxonomy field that "This field does not save any value in the database"?? Where is it saved? About the Taxonomy Advanced field you say: "store terms’ IDs in the post meta and doesn’t set post terms". I don't understand these two explanations. Can you further explain this and especially how they relates to custom tables (is anything saved there?)?
The field taxonomy sets the post terms, the value is saved to the table wp_terms in the database.
The field taxonomy_advanced sets the post meta, the value is saved to the table wp_postmeta in the database.
You can see more tables of WordPress here https://codex.wordpress.org/Database_Description
So if you use the custom table to store field value, you should use the field taxonomy_advanced.
The data type TEXT can be used for most cases. If you want to change the data type later, you can access your database and change it or contact your hosting support to ask to change the data type. Please read more here https://www.w3schools.com/sql/sql_datatypes.asp