References, Tags, and Portals are all ways of using the same content in multiple places and defining how things relate to each other. However, they have distinct functions.
Summary
Suppose you have two bullets, creatively called A and B, and you're editing A and want to refer to B.
| Reference | Tag | Portal |
In a few words | A link to another bullet | A note that A goes in category B | A window showing another bullet |
Implies that | B is related to A | A is a type of B | B is used in the context of A |
How to create it | Type | Type | Type |
What it looks like |  |  | 
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Everything it does |
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Full docs |
References
A Reference is simply a link to another bullet. The name of the bullet you reference appears in blue, and clicking on it takes you to the referenced bullet.
Use References when you want to mention some other related concept or piece of information, but don't think you'll need to immediately revisit its definition when rereading the notes later. If you think you will need the definition, you might prefer a Portal instead for greater convenience.
There's one additional trick to References: In RemNote, all links are bidirectional. That is, if you create a reference from Keyboard to Computer, you can not only see this link when looking at the Keyboard bullet, but also when looking at the Computer bullet. This backwards link (often called a backlink) appears under a References section which you can access by clicking the numbered bubble to the right of the Computer bullet.
The simplest way to create a Reference is to press [[ while typing; this will bring up an inline search box which you can use to find the bullet you want to reference.
Tags
Tagging one bullet with another means that the tagged bullet is a type of the tagging bullet. To make that more concrete, a Keyboard and a Mouse are both types of Hardware, so you might apply the Hardware tag to the Keyboard and Mouse bullets.
As with References, tags are bidirectional; you can see all of the places a tag is used by clicking the numbered bubble on the right side of that tag's bullet.
The simplest way to add a tag to a bullet is to press ## while typing; this will bring up an inline search box which you can use to find the bullet you want to tag the current bullet with.
Portals
A Portal displays a particular bullet, and optionally some or all of its descendants, at an arbitrary place within some other notes. You can think of a portal like a window cut in the page with the source document placed behind it: you can see all the material at the other location right within the current document, but it's actually located in a different document, and if you update the information in one place, it will be updated in the original location as well as all its portals.
Use Portals when you want to keep information that is relevant in many contexts in one centralized place, but include it directly within your notes on particular sources or more specific topics for easy review.
You can tell that content is part of a portal when there is a blue bar at the left side of the content. When you click inside the portal, the bar extends into a full border around the entire portal.
Again, portals are bidirectional; the numbered bubble will show any places where the current bullet has been included in a portal (unless the portal is within the current document).
The simplest way to add a portal is to press (( while typing; this will bring up an inline search box which you can use to find the bullet you want to tag the current bullet with.
Note: In other notes systems, features similar to portals may be called transclusions or mirrors.