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So the content of the iframed document does not "count" as part of the parent page. You need to treat that iframed document like the independent url that it is, technically. This includes giving it a unique, page specific title element and meta description. Also make sure it's on the sitemap or has other inbound links in addition to the iframe src attribute, which isn't a "true" link.
Even more, assume that your iframed url begins to gain some search engine traffic. Then visitors from search results will arrive at the "supposed-to-be-iframed" url without the parent page to surround it, give it some context and orientation for the new visitor.
So you need to consider how you will give that traffic the kind of comfortable experience that leads them on into the rest of your website. Lots of technically different approaches are possible here, mostly involving some kind of scripting. The key is making sure you take some step.