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“Outlook data types” can refer to two different things: files that hold or synchronize mailbox data, and formats for individual items such as messages, calendar events, and contacts. For mailbox data, the key distinction is PST versus OST: a PST can serve as an archive or export, while an OST is a local synchronized copy for supported accounts. MSG, ICS, VCF, and CSV are used for individual items or contact exchange—not as mailbox containers.
Outlook data types at a glance
Outlook does not have one universal file format for every kind of data. A file’s purpose depends on whether it stores a folder-and-message collection, keeps account data available offline, or represents a single item for saving or exchange.
| Format | What it represents | Typical role |
|---|---|---|
| PST | A message store with a folder hierarchy, messages, and attachments | Archive or export; also used for POP account storage |
| OST | An offline Outlook data file containing a synchronized copy of server information | Local working copy for supported account types |
| MSG | One Outlook item, such as an email, appointment, contact, or task | Save an individual item as a file |
| ICS | A calendar item | Calendar-event file and interchange |
| VCF | A contact item | Contact file and interchange |
| CSV | Contact information arranged in rows and columns | Contact import or export; not a mailbox store |
What is an Outlook data file?
In the narrower sense, an Outlook data file is a local file used to store or make account information available in Outlook. PST and OST are the main formats to recognize. Microsoft’s PST guidance describes their roles across Outlook 365, Outlook 2024, 2021, 2019, and 2016, while the exact arrangement depends on account type and setup.
PST: a store for folders and messages
A PST (Personal Folders) file can contain a hierarchy of folders, messages, and attachments. Microsoft documents it for archives and exports, and as storage for POP accounts. The [MS-PST] specification defines the PST format as a message store. A PST is therefore useful when the task is keeping or moving a collection of Outlook data, rather than saving just one email.
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OST: a synchronized offline copy
An OST (offline Outlook data file) is used by supported account types to keep a synchronized copy of server information available in Outlook. It is not equivalent to a portable archive: Microsoft says an OST can be synchronized again from the server and does not need to be backed up as a standalone copy. Whether Outlook uses an OST or PST depends on the account category and configuration; do not assume every Outlook account has the same file arrangement.
What are MSG, ICS, VCF, and CSV files?
These formats describe individual items or contact data, rather than a whole mailbox store.
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MSG: one Outlook item in a file
An MSG file represents an individual Outlook item, including an email, appointment, contact, or task. Microsoft’s [MS-OXMSG] specification defines the format for individual Outlook Message objects. Use this distinction when you need to identify a saved item file rather than a mailbox archive.
ICS: calendar items
ICS is associated with calendar items. It is the relevant format to recognize when the file represents a calendar event rather than a message store. Microsoft’s MsoFileType enumeration maps Outlook item types to extensions including ICS.
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VCF and CSV: contact exchange
VCF is associated with contact items. CSV stores contact details as tabular data, which makes it a different kind of exchange format from a VCF or a PST. Microsoft’s import and export instructions cover CSV and VCF contact workflows as well as PST transfers. The available options and steps can vary by Outlook version and account setup.
Which Outlook format should you use?
- To keep or export a collection of mail and folders: consider a PST, and confirm it matches your account and intended transfer.
- To understand a local file used by a connected account: an OST is likely a synchronized offline copy, not an archive to move between systems.
- To save one email, appointment, contact, or task: look for MSG.
- To exchange a calendar item: look for ICS.
- To exchange a contact: VCF is a contact-item format; CSV is a tabular contact import/export option.
If you need to move data, choose the format based on the content and destination, then follow Microsoft’s version-appropriate import/export steps. An OST should not be treated as a portable backup simply because it is stored locally.
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Technical specifications and terminology
For software developers or administrators implementing format support, Microsoft publishes the PST and MSG specifications and maps supported item types to extensions in its MsoFileType documentation. Outlook and Exchange standards documentation also covers related standards such as iCalendar, IMAP4, and POP3 in the Microsoft standards documentation download.
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