Subscriptions works by matching metadata defined in the Subscriptions Setup - Configuration with metadata values related to newly created articles.
To give the metadata matching more focus (ie so you don't create subscriptions alerts every time you use a certain metadata value), the articles that will be considered for alerts also have to be created beneath certain parent articles, also defined in the configuration. The configuration is also organised by subsite.
Creating Articles
This example shows a simple news article subscription. Users can subscribe to news topics and to certain locations.
The metadata that can be used to tag articles looks like this:

In the configuration the metadata properties are part of a group in the GOSSDocs subsite:
GOSSDocs: [{For an article to be considered for a subscription alert, it would have to be created beneath article 10075 on the GOSSDocs subsite, and have one or more metadata values from either of the two properties related to it.
groups: [],
properties: [{
title: 'Locations'
property: 1765
}, {
title: 'Topics',
property: 1766
}],
parent: {
id: 10075,
title: 'Latest News Articles'
}
}]
Matching Logic
The form used to manage subscriptions lets users pick the subjects they are interested in. The checkboxes on the form correspond to the metadata values in your configured metadata properties:

See Subscriptions Setup - Creating the Manage Subscription Form.
The logic by which an article is deemed to match a subscribed user's chosen topics of interest (and, therefore, determines its inclusion in alert emails), can be summarised as: OR between values, AND between properties, OR between groups.
Using the example subscription choices above, an article would have to be tagged with "Plymouth" and either "Events" or "Travel".
If a user does not check any options in a particular property (eg hadn't picked "Events" and "Travel" above), then that property would not be included in the logic, meaning alerts would be sent for "Plymouth" articles regardless of whether they have related values from the "Topic" property or not. Similarly, if an article is not tagged with any values from a particular property, it would only match for users who also hadn't picked any checkboxes under that property.
