So we have these nurturing campaigns that are always on - or our evergreen campaigns. The concern is when we have content that is specific to a segmented group within the nurturing campaign. How do we target and email these specific leads that are currently in a stream?
One way is to create another stream within your engagement program. Click the add stream button along the top, and in your new stream, select transition rules. Input any filters that help you segment out the leads you want, like industry or sub-industry, and voila.
We do this for a few different verticals, like healthcare. There's a basic track that's super general, but if we know whether they're a hospital, senior living community, or pharmacy, they automatically get switched to the right track.
Alternatively you could use dynamic content and have two versions of the email, one for the specific segment and one for the rest audience. More information here: Understanding Dynamic Content - Marketo Docs - Product Docs