Many2manyCustom field is useful when a direct access to the relational table
is needed, for example to be editable in a dedicated tree view.

Let's consider following models:

.. code-block:: python

    class MyModelA(models.Model):

        _name = 'my.model.a'

        my_model_b_ids = fields.Many2manyCustom(
            'my.model.b',
            'my_model_a_b_rel',
            'my_model_a_id',
            'my_model_b_id',
            create_table=False,
        )


    class MyModelB(models.Model):

        _name = 'my.model.b'

        my_model_a_ids = fields.Many2manyCustom(
            'my.model.a',
            'my_model_a_b_rel',
            'my_model_b_id',
            'my_model_a_id',
            create_table=False,
        )


    class MyModelABRel(models.Model):

        _name = 'my.model.a.b.rel'

        my_model_a_id = fields.Many2one(
            'my.model.a',
            required=True,
            index=True,  # Index is mandatory here
        )
        my_model_b_id = fields.Many2one(
            'my.model.b',
            required=True,
            index=True,  # Index is mandatory here
        )


By setting `create_table=False` on the Many2manyCustom field, and using the
relational table name, as `_name` for the relational model, we're able to
define a dedicated tree view for `my.model.a.b.rel`.

.. code-block:: xml

    <record id="my_model_a_b_rel_tree_view" model="ir.ui.view">
        <field name="name">my.model.a.b.rel.tree.view</field>
        <field name="model">my.model.a.b.rel</field>
        <field name="arch" type="xml">
            <tree editable="top">
                <field name="my_model_a_id" />
                <field name="my_model_b_id" />
            </tree>
        </field>
    </record>