> For the complete documentation index, see [llms.txt](https://docs.os.uk/os-downloads/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.os.uk/os-downloads/products/land-and-terrain-portfolio/os-mastermap-sites-layer/os-mastermap-sites-layer-technical-specification/data-specification-overview.md).

# Data specification overview

## Structure <a href="#bookmark5" id="bookmark5"></a>

This data specification works within the existing structure of OS MasterMap as represented in the Unified Modelling Language (UML) class diagram below:

<figure><img src="/files/GNhSqXjNJaZ1m4D28koC" alt="UML class diagram representation of OS MasterMap layers"><figcaption><p>UML class diagram representation of OS MasterMap layer</p></figcaption></figure>

## Layers <a href="#bookmark6" id="bookmark6"></a>

A layer is a set of related geospatial data which is then divided into one or more themes (much like OS MasterMap Topography Layer). These themes can be used together to form an end-user application.

## Themes <a href="#bookmark7" id="bookmark7"></a>

Within the Sites Layer, features belong to only one theme. A theme is a logical collection of features that have been grouped according to their classification or relationships. The primary purpose of themes is to enable easier selection and interrogation of features by the user. Themes do not form part of the classification of a feature and do not affect the feature life cycle rules. The Sites Layer is currently composed of seven themes which are defined fully on the [Theme definitions](/os-downloads/products/land-and-terrain-portfolio/os-mastermap-sites-layer/os-mastermap-sites-layer-technical-specification/theme-definitions.md) pages.

## Features <a href="#bookmark8" id="bookmark8"></a>

Features are digital representations of real-world concepts such as a building, road or barrier. The life cycle of a feature, its creation, modification and deletion are managed to most appropriately reflect the life cycle of the abstracted real-world concept that they depict.

## Attributes <a href="#bookmark9" id="bookmark9"></a>

An attribute is any item of information contained within an OS MasterMap feature. The TOID and the geometry of the feature are both attributes.


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