An STS license allows the member the ability to use the STS Logo and be illegible for certification of his product. It also permits access to various companion specifications the the STSA publishes from time to time, the contants of which may not (yet) be published in the IEC specifications. Any product that displays the STS logo may only do so if the manufacturer has an STS license, i.e. is a member of the STSA.
If a company wishes to have an STS certified product and display the STS logo, it must be a paid up licensee or member of the association. Your company will need to pay an annual subscription fee to the association to retain the right to use the STS logo and maintain its certification.