Tags are very similar to Refs with one very important distinction: Tags work across multiple sources.
In Mosaic, each source has it's own filesystem which are then merged together to form a union of all source filesystems. It is in this union filesystem that tags are applied and not to the individual source filesystem that the tag was defined on.
Tags are slower to apply than refs. Multiple sources need to run and update before tagged data will be resolved. If possible, stick to refs and only use tags when dealing with multiple sources.
To tag a page, add a tags property to the page frontmatter. For example, the Product A page is tagged with "in-stock":
tags is always an arrayTo subscribe to a tag, use the $tag property. For example, the Products page has subscribed to the data property of pages tagged with in-stock.
A $tag can provide a path to a specific piece of metadata on tagged pages, just like a ref.
This page has subscribed to in-stock and out-of-stock tags and is displaying them using 2 Mosaic PageFilterView components.
Both the Product A and Product B pages are part of a different source than this page.