Although a field's ReadOnly property may be False, it will behave as if it is True if the field is based on another field and that field's ReadOnly is set to False. The EffectiveReadOnly property allows you to get this effective value. A table view field is based on a table field if the table view field's TableField property is not empty. A composite table field is based on a table field if the composite table field's TableViewField property is not empty and TableViewField.TableField is not empty.

Reference

Field Class
Field Members
Table Fields