Any organization/firm create a document which used to ordering goods, it is prepared by buyer (i.e. firm/organization) to sent the suppliers. So creation of these documents easily and keep the records of every document using this section.

Before creating Purchase Order you mast have to add Supplier first.
Click on Commerce > Supplier
Click on Commerce > Purchase Order Menu :
You will get page:

Basic Detail Section :

Supplier Name : Please select supplier name.
Address : Fill address details.

Purchase Order No : It automatically generated when we create any new purchase order.
Create Date : Mention creation date, by default it takes present day's date.
Due Date : Expected date for receiving of order.
Terms & Condition : Please select terms & condition for particular purchase order.
Currency : Please select currency.
Add Item : After addition of basic details, click to 'Add Item' to add in purchase order.

Item Details :

Item : Select an item.
Taxation : Select tax to apply on item. 
Quantity : Mention quantity.
After successfully insertion of record,  We got Purchase Order detail here.

Actions And Their significant :
On almost every grid in epan, you will see Status (Active/InActive or Draft etc.) and action dropdown. 
In purchase order there are multiple kinds of actions based on status :

1.  On Draft :
   [ Submit, Manage Attachments ]
   Submit : For submitting document for approval by higher authority.
   Manage Attachments : For manage new attachments with this document.
2. On Submitted :
   [ Reject, Approve, Manage Attachments, Print Document ]
   Reject : For reject this document.
   Approve : For approve this document, its mean this order is in approved by the supplier to dispatch.
   Print Document : For print doc.
3. On Approved :
   [ Reject, Redesign, In Progress, Create Invoice, Send, Manage Attachments, Print Document ]
   Reject : For reject the order.
   Redesign : For redesign this document to place an order.
   Create Invoice : For creation of the purchase invoice using this order detail.
   In-Progress : For proceed to order in progress.
   Send : For send this document to supplier.
   Note : Please select 'From Email' , using which email want to send email.
4. On In-Progress :
   [ Complete, Send, Cancel, Manage Attachments, Print Document ]
   Complete : For mark complete this order after successfully completion.
   Cancel : For cancellation of order.
5. On Redesign :
   [ Submit, Manage Attachments ]
6. On Canceled :
   [ Redraft, Manage Attachments ] 
7. On Rejected :
   [ Submit, Redesign, Manage Attachments ]
8. On Partial Complete :
   [ Complete, Send, Manage Attachments ]
9. On Completed :
   [ Send, Manage Attachments, Print Document ]
   You will get this window :
   
   After click on Complete Purchase Order button receive item will be added in stock automatically.
   You can complete your order Partial Complete/Full complete.

After successfully creation of Purchase Order, Now you have to further move to 'Create a new Purchase Invoice'.