Hi Pascal,

you can find more information about inheritance in the doc...

https://www.odoo.com/documentation/10.0/reference/orm.html#odoo.models.Model._inherits

https://www.odoo.com/documentation/10.0/reference/orm.html#inheritance-and-extension

As you can see, this field is declared on res.partner: https://github.com/odoo/odoo/blob/10.0/odoo/addons/base/res/res_partner.py#L177

And on res_users, you have the partner_id whith delegation inheritance (inheritS): https://github.com/odoo/odoo/blob/10.0/odoo/addons/base/res/res_users.py#L175

class Users(models.Model):
    _name = "res.users"
    _inherits = {'res.partner': 'partner_id'}

As explain, with inherits: "implements composition-based inheritance: the new model exposes all the fields of the _inherits-ed model but stores none of them: the values themselves remain stored on the linked record."

so if partner has field 'foo'.

user.foo ====> user.partner_id.foo