# APIs: Common Keys

_(c) AMWA 2018, CC Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0)_

A number of common keys are used within NMOS APIs. Their definitions, required practices for interpretation and permitted values are described below.

## Version

Core resources include a 'version' attribute. As properties of a given resource will change over its lifetime, the version identifies the instant at which this change took place.

The version is a nanosecond TAI timestamp represented as a string of the format: &lt;seconds&gt;:&lt;nanoseconds&gt;. Note the ':' as opposed to a '.', indicating that the value '1439299836:10' should be interpreted as 1439299836 seconds and 10 nanoseconds.

Combining the version with the resource's ID attribute creates a hash which can be used to uniquely identify a particular variant of a resource.

## Label

Freeform human readable string to be used within user interfaces.

## Description

Freeform human readable string to be used within user interfaces.

## Tags

A set of keys and values providing a means to filter resources based on particular categorisations. Each tag has a single key, but MAY have multiple values. Each tags SHOULD be interpreted using the comparison of a single key value pair, with the comparison being case-insensitive following the Unicode Simple Case Folding specification.

**Example: Tags Format**
```
{
  "tags": {
    "location": ["Salford", "Media City"],
    "studio": ["HQ1"]
  }
}
```
