Microsoft Bing has published a document on SEO best practice for subscription-based paywall content with the aim of getting more visitors from search without affecting the publisher's subscription model.
There's a two stage best practice, and the first of which is to enable crawling of the content behind the paywall.
Stage two is a little more detailed, and includes the important aspect of avoiding "leaking subscription-based or paywall content in search results cache pages." It includes use of Robots Meta tag, or x-Robots- Tag, and setting it to noarchive, or nocache.
More detailed info is here
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blogs.bing.com...]
