Wanted Delivery Date
This is the date on which the supplier has agreed to deliver the parts. This date will be printed on documents and will be sent to the supplier.

Usually it is derived from the Planned Arrival Date. The formula for calculating the wanted delivery date is as follows:  
Wanted Delivery Date= (Planned Arrival Date - Transport Lead Time). 
You can change this date manually, e.g., when you want to coordinate several deliveries from the same supplier. 

Planned Arrival Date
This is the date on which the parts are expected to arrive at the site's arrival location.  If no delivery route is specified, planned arrival date is calculated as follows: 
Planned Arrival Date = (Planned Receipt Date -Internal Inspection Time - Internal transport Lead Time) 
If a delivery route exists, then select the latest date and time before that date from the specified delivery route.
 

To answer your question,

The delta between “wanted delivery date” and  “planned arrival date” would be the “Transport Lead Time”

As I think, from above 2 dates changing the “planned arrival date” is more preferred. This is because “wanted delivery date” get recalculated when you changed the “planned arrival date”.

Suppose this answer will help you some way.

Thank you Anushka.  Which date or dates effects the MRP date for planners to see?  I want to be able to change that one without effecting my original want date. What fields would I use to create a supplier on time delivery report based on the actual date I want it versus the actual date it arrived?

When measuring Supplier Reliability, system let the user to compare Promised delivery date with Delivery date.

Promised delivery date: 
The date that the supplier has agreed or confirmed to deliver to an agreed location. 
At the early life of the purchase order we assume that it is the same as our wanted delivery date. 
At a later stage, after receiving the order confirmation, it will represent when the supplier has promised to deliver the parts to the agreed location.

Delivery date: 
This is the date when the parts were actually delivered at the delivery address.

Similarly, delivery reliability of customer orders is measured comparing promised delivery date to actual delivery date. 
 

So, I hope you can analyse supplier on time delivery performance by comparing Promised delivery date and Delivery date

@Anushka Bandara @Rangi Wijewardana @Trish453 I found this thread really useful when I was reading through. Based on the statistics of these delivery dates, I am trying to figure out how to build a Supplier Performance Measurement Matrix in IFS. Are there any Quick reports, etc that you are familiar with, that you might be using? 

One could be Supplier Reliability Statistics @Rangi Wijewardana mentioned above. Anything else?