What and where is the subtask mentioned in negotiation approvals online document

Summary:

In Negotiation approval process, there is statement mentioned which conveys about subtask available in negotiation line and based on the line subtask, approvals can be derived. But how to enable and what is the significance of it is no where mentioned. Below is the statement and link where it is mentioned:

For a particular negotiation item, the category manager requires some information from a subject matter expert, the category manager creates a subtask from the Actions menu and assigns the subtask to the subject matter expert for information and an approval.