The companion formats

Three companion formats are single XML files in IDML's vocabulary; the fourth, the library, is a binary container. None of them is a package.

In short An IDML package is a ZIP archive with a mimetype, a designmap.xml and the parts the design map names. An InDesign snippet (.idms), an InCopy story (.icml) and an InCopy assignment (.icma) are each one XML file, with no archive and no design map, identified by the <?aid ?> processing instruction at the top. A library (.indl) is InDesign's own binary file for reusable items. This page states each as a fact.

How a companion file identifies itself

Like a design map, a companion file opens with an <?aid ?> processing instruction (see version markers). Its type pseudo-attribute says what kind of file it is, where a package's design map says document. Snippet and story files carry a second instruction naming the kind of snippet.

These markers are recorded from files commonly seen in the wild. Our corpus holds no companion files, so they have not been checked against InDesign's output for this reference.

InDesign snippets (.idms)

A snippet holds a selection of page items, with their text and the styles, colours and layers needed to rebuild them when the snippet is placed into a document. It uses the element names of the package's spread, story and resource parts.

How it differs from a package: there is no designmap.xml and no ZIP container. Nothing names a document root or lists the document's stories, and nothing sets document-wide preferences. The snippet's meaning is completed by the document it is placed into, which decides what happens to incoming styles and colours whose names it already uses.

InCopy stories (.icml)

An ICML file holds one story, its text and formatting, plus the styles it uses. It is the file InDesign and InCopy exchange when a story is edited separately from the layout.

How it differs from a package: there is no spread, so no frame, no geometry and no page. In a package a story is reached through the design map's StoryList and laid into frames that name it in ParentStory (see content parts). An ICML story has nowhere to flow until it is linked back into a layout.

InCopy assignments (.icma)

An assignment groups stories into a unit of work for one editor and records which stories belong to it.

How it differs from a package: it describes work on a document rather than the document. It refers to stories rather than standing alone as something to lay out.

Libraries (.indl)

A library is InDesign's file for keeping reusable items across documents: a masthead, a pull-quote treatment, a logo. Its items behave like snippets when you drag them into a document.

How it differs from a package: it is not an XML format at all. Its internal structure is not described in this reference.

Side by side

How each one turns up in practice is on when each applies.

In Paged: none of these files can be opened; Paged's IDML importer reads .idml packages and .paged containers only. Planned

Frequently asked questions

What is the difference between an .idms snippet and an .icml story file? A snippet holds page items, with their geometry and the resources they need, ready to be placed into a document. An ICML file holds one story's text and formatting with no page geometry at all. One is a piece of layout; the other is text without a place to sit.

How can I tell which companion format an XML file is? Read the <?aid ?> processing instructions at the top. type="snippet" with SnippetType="PageItem" marks a snippet; type="snippet" with SnippetType="InCopyInterchange" marks an InCopy story; type="assignment" marks an assignment. A package's design map says type="document".

Is an InDesign library made of snippets? Its items act like snippets when placed, but the library file (.indl) is InDesign's own binary format, not a collection of XML files.

Can Paged open any of these files? No. Paged opens IDML packages and .paged containers. There is no reader for snippets, InCopy files, assignments or libraries.