When a workflow is embedded in an activity (as a subflow) of a workflow, the sublow can send a signal from its own activities to the parent workflow by giving a signal name in the attribute signal_send. Odoo processes those activities by sending the value of signal_send prefixed by "subflow." to the parent workflow instance. For example see below code.

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <record model="workflow" id="wkf_academy">
                <field name="name">Academy.wkf</field>
                <field name="osv">session</field>
                <field name="on_create">True</field>
        </record>

Note-The activity is considered completed (and its outgoing transitions ready to be evaluated) when the subflow is completed.