If your model has too many fields, then you can use the <notebook> and <page> tags to create tabs. Each <page> in the <notebook> tag will create a new tab, and content inside the page will be the tab content. The following example, it will create 2 tabs with 3 fields in each tab:

<notebook>
    <page string="Tab 1">
        <field name="field1"/>
        <field name="field2"/>
        <field name="field3"/>
    </page>
    <page string="Tab 2">
        <field name="field4"/>
        <field name="field5"/>
        <field name="field6"/>
    </page>
</notebook>

The string attribute in the <page> tag will be the name of the tab. You can only use <page> tags in the <notebook> tag, but in the <page> tag, you can use any other elements.