AS9100 Rev D additional purchasing requirements

Answer:
Since you have already updated to ISO 9001:2015, there are several additional requirements for purchasing in clause 8.4, control of externally provided processes, products and services. They use the term external providers to make it clear that the requirements apply to outside providers, and not just suppliers or products. However, I will use suppliers in my answer to make things easier.
To start with, the added requirements talk about using customer-designated or approved suppliers when the y are designated, and highlighting that you are responsible for the conformity of everything that comes from your suppliers even if the customer told you who to use. You need to identify and manage the risks of suppliers (including those during selection of suppliers), and require that you have your suppliers flow down necessary requirements to their suppliers so that all requirements are met.
Clause 8.4.1.1 is added to ensure that you have a process for approval and maintenance of suppliers, including review of their performance and on-time delivery, what actions you take to deal with suppliers that don’t meet requirements and how you control documents sent to you by suppliers. Clause 8.4.2 adds control requirements, including periodic testing when there is a high risk of counterfeit parts, having a process for positive recall if you release parts before all verification is completed, managing delegation of verification to suppliers and having a process to evaluate test reports from suppliers if these are used.
There are finally some additions to clause 8.4.3 on the information to send suppliers. This includes information on design control, special requirements (key characteristics and critical items), test and verification activities, use of statistical techniques, and right of access for customers and regulatory agencies where applicable. Other requirements to give suppliers include having a QMS, customer-designated sources, notification of nonconforming products, not using counterfeit parts, flow-down of customer requirements, the need for test specimens and how long to retain records. You must also ask that supplier employees are aware of how they contribute to conformity, product safety and ethical behaviour.
For a closer look at the AS9100 Rev D requirements in a simple English explanation, see the whitepaper: Clause-by-clause explanation of AS9100 Rev D, https://info.advisera.com/9100academy/free-download/clause-by-clause-explanation-of-as9100-rev-d

Thank you very much for the response Mark. The information that you had provided was very informative.
I do have one other question. In your statement, "To start with, the added requirements talk about using customer-designated or approved suppliers when the y are designated, and highlighting that you are responsible for the conformity of everything that comes from your suppliers even if the customer told you who to use". I don't understand what the "Y" is. Could you please advise. Thanks

Disregard previous question.