Activity Streams/Primer/Complex Properties

Many of the terms in the Activity Streams 2.0 namespace have a @type of @id. There is a full list at the end of this page.

This means that the property value can be a first-class object in the AS2 system.

The possible values for these properties in an AS2 document are surprisingly varied. Here are some possibilities:

An ID reference, represented as a string. This is the ID of the related value. It is an URI, either fully defined such as "acct:username@example.com", or a shortened version of the URI as a curie (AKA compact URI), like "as:Public", or an unprefixed term, like "Public". In ActivityPub, these IDs should be a resolvable HTTPS URI.
An object (AKA hash). This is a representation of some or all of the properties of the object.
An array. Unless otherwise noted, any @id property can have multiple values. These will be represented in an array. The elements of the array are commonly strings, objects, or a mix of the two.
A literal value. For a few properties, such as closed, the value can alternate between one of the above, or a literal value like a datetime.

Guidance for publishers

The contents of @id properties should either be a resolvable HTTPS URI if possible, or an object.

In object format, the value should include at least the id, type, and one of name, nameMap, summary, summaryMap.

For values in arrays, it is easier for consumers to process array items that are of a uniform type.

For arrays of a single element, instead include that element directly. So, use "type": "Note" instead of "type": ["Note"].

Be ready to process array values for all of these properties. In particular, the type value can be an array.

TBD