For Purchase Orders and Credit Notes an Approval can be requested. An Approval Task will be created and the document will be 'Locked for Processing' until the Approval Task is completed.
To do this firstly open or create either a Purchase Order or Credit Note.
Click the 'Request Approval' button. 
A warning message will pop up reminding you that the document will be 'Locked' until the Approval has been completed. Click 'Yes' and the Approval Task will be created.
You can find the Approval Task by navigating to 'Tasks' > 'Processing' >'My Approvals'/'All Approvals'. My Approvals will only show approval tasks assigned to you.
Alternatively, you can find the approval tasks for a specific document by opening the document and navigating to the 'Approvals' tab.
Based on the Approval Task Template, this credit note received 2 approval tasks. To create an Approval Task Template, navigate to 'Documents' > 'Maintenance' > 'Approval Task Templates'.
Click the 'New Approval Task Template' button. 
Provide a 'Name' and a 'Description' for the Approval Task Template. On the 'Template Tasks' panel you can add task lines. Defining 'Approval Level Amount' means that the approval task will be assigned if the transaction being approved has a Total value greater than the Approval Level Amount. The 'Assign To' field tells us who is responsible for approving the transaction. Otherwise 'Assigned To' is set to you by default.
Click 'Save' 
to save the template.