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Configuration settings can also have fields named with the default_ prefix. When one of those has a value, it will set it as a global default. The settings field should have a default_model attribute to identify the model affected, and the field name after the default_ prefix identifies the model's field that will have set the default value.

Additionally, fields with none of the three prefixes mentioned can be used for other settings, but you will need to implement the logic to populate their values, using get_default_ name prefixed methods, and to act when their values are edited, using set_ name prefixed methods.

For those who would like to go deeper into configuration settings details, the best documentation is in Odoo's source code in  ./odoo/addons/base/res/res_config.py, which is massively commented on with all details explained.