It is concerned with the Mission (where are we now?), Vision (what is longer term goal?) and the market positioning (why use us? for this?).
The positioning statement represents the product/market field of play. Here we are concerned with what the customer wants, what we are selling versus what the customer is buying?

The process of setting the proposition, termed the Unique Selling Proposition, provides an opportunity to gain a major insight to the company and its approach to the market.

The proposition statement arrived at will reflect back into the offering of the company to the market and the delivery of the company to its customers.