what is the usage of these field in xml odoo?

<field name="category_id" ref="employee_document.module_category_employee_document"/>
<field name="implied_ids" eval="[(4, ref('group_creat_employee_document_order'))]"/>
<field name="users" eval="[(4, ref('base.user_root')), (4, ref('base.user_admin'))]"/>

I had a button not appears even the group privilige is given, I tried to add <implies_id> and it worked after that and the button shown, I didn't understand why and what those fields represents