There are four components to an eCTD:

• Scientific and technical contents;
• Folder structure;
• XML backbone;
• Study Tagging Files (US only).

An eCTD consists of discrete "sequences": the initial submission is sequence 0000, and subsequent amendments and supplements are provided as sequences 0001, 0002, 0003 etc. Each sequence has its own folder structure, XML index file (backbone), contents and study tagging files.

Scientific and technical contents

The scientific and technical contents of an eCTD are usually files in Acrobat (PDF) format, although other formats can be used e.g JPEG, PNG, SVG or GIF format for images. PDF is "portable document format", a proprietary file format for documents developed by Adobe Systems Inc. The viewer software for PDF (Acrobat Reader) is distributed free by Adobe, and there are versions for all operating systems is common use. Thus, documents distributed in PDF can be read by anybody, irrespective of the type of computer that they use or their operating system. PDF documents can be bookmarked and hyperlinked, for ease of navigation and review. The PDF format is also relatively compact, thus conserving disk space and reducing download/upload times. PDF also has useful security features, including support for digital signatures.

Typically, a complete CTD or eCTD dossier contains many thousands of pages. It is not practical or reviewer-friendly to submit an eCTD/CTD as a single document, and therefore it is necessary to break up the dossier into multiple documents. The Annex to the ICH M4 guideline provides detailed guidance as to how this should be achieved. The term "granularity" is used to describe the presentation of a CTD/eCTD as a collection of discrete documents.

In a paper CTD, a document is defined in the ICH M4 Annex as "a set of pages, numbered sequentially and divided from other documents by a tab". The ICH M4 Annex adds that "a document can be equated to a file for an electronic submission".

In general, ICH M4 specifies the coarsest granularity that is considered acceptable. Often, the applicant has the option to choose a finer granularity.

In a complete eCTD:

• Module 2 will be broken down into at least 17 discrete documents;
• Module 3 will be broken down into 50+ discrete documents;
• Module 4 study reports are usually presented as one discrete document per report;
• Module 5 study reports are usually broken down into multiple discrete documents per report (a "synopsis", a "body" and multiple "appendix" documents).

PDF documents within an eCTD will contain extensive bookmarking and hyperlinking, to facilitate navigation through the dossier and ease of review.

Folder structure

The eCTD folder structure follows the CTD hierarchical structure. It is essentially just a container for the files that make up the scientific content.

XML backbone

The XML index file (backbone) is effectively an "inventory" or "catalogue" of the submission contents. When opened in a web browser e.g. Microsoft Internet Explorer, it displays as a hyperlinked table of contents for the submission. It also contains version control information to support life cycle management.

Study Tagging Files

Study Tagging Files contain descriptive information on nonclinical and clinical studies in Modules 4 and 5. They are required only in the USA.