Content types in SharePoint can be extended by creating a new feature which deploys an element file with the following content:

<Elements xmlns=”http://schemas.microsoft.com/sharepoint/“>
    <ContentType ID=”GUID
        Name=”Text
        Group=”Text”
        Version=”0″>
        <FieldRefs>
            <FieldRef ID=”GUID” Name=”Text“/>
        </FieldRefs>
    </ContentType>
</Elements>

The content type Id includes the GUID of the Parent content type. The base content type (System) has an Id of 0x. The Id is built up as follows:

Option 1

[ParentGUID][01]
[ParentGUID][02]
[ParentGUID][03]

Option 2

[ParentGUID]00[GUID]

Out of the box examples for the first format are as below:

Content Type GUID Inherits from Comment
System 0x
Item 0x01 System
Document 0x0101 Item
Form 0x010101 Document In the resource files this is refered to as an XML Document
Picture 0x010102 Document
Untyped Document 0x010104 Document
Unknown Document Type 0x010105 Document Also called untyped Document
User Workflow Document 0x010107 Document
Wiki Page 0x010108 Document Also called Wiki document
Basic Page 0x010109 Document
Web Part Page 0x01010901 Basic Page
Link to a document 0x01010A Document
Far East Contact 0x010116 Document
Dublin Core Columns 0x01010B Document
Event 0x0102 Item
Issue 0x0103 Item
Announcement 0x0104 Item
Link 0x0105 Item
Contact 0x0106 Item
Message 0x0107 Item
Task 0x0108 Item
Workflow Task 0x010801 Task
Admin Task 0x010802 Task
Workflow history 0x0109 Item
Blog post 0x0110 Item
Blog Comment 0x0111 Item
Folder 0x0120 Item
Discussion 0x012002 Folder
Summary Task 0x012004 Folder
Document Set 0x0120d5 Folder
Person 0x010A Item
SharePoint Group 0x010B Item
Domain Group 0x010C Item

Examples of the second format are:

Content Type GUID  
Office Data Connection File 0x010100629D00608F814dd6AC8A86903AEE72AA
Universal Data Connection File 0x010100B4CBD48E029A4ad8B62CB0E41868F2B0
Health rule definition 0x01003A8AA7A4F53046158C5ABD98036A01D5
Health report 0x0100F95DB3A97E8046b58C6A54FB31F2BD46
Schedule 0x0102007dbdc1392eaf4ebbbf99e41d8922b264
Resource Reservation 0x0102004f51efdea49c49668ef9c6744c8cf87d
ScheduleAndResourceReservation 0x01020072bb2a38f0db49c3a96cf4fa85529956
GbwCirculationCTName 0x01000f389e14c9ce4ce486270b9d4713a5d6
GbwOfficialNoticeCTName 0x01007ce30dd1206047728bafd1c39a850120
CallTracking 0x0100807fbac5eb8a4653b8d24775195b5463
Resource 0x01004c9f4486fbf54864a7b0a33d02ad19b1
ResourceGroup 0x0100ca13f2f8d61541b180952dfb25e3e8e4
Holiday 0x01009be2ab5291bf4c1a986910bd278e4f18
Timecard 0x0100c30dda8edb2e434ea22d793d9ee42058
WhatsNew 0x0100a2ca87ff01b442ad93f37cd7dd0943eb
Whereabouts 0x0100fbeee6f0c500489b99cda6bb16c398f7
IMEDictionaryItem 0x010018f21907ed4e401cb4f14422abc65304
XSL Style 0x010100734778F2B7DF462491FC91844AE431CF

Which format should be used?

SharePoint uses the long GUIDs when you create a new content type however it does mean that you might end up with very long set of characters making it almost impossible to manage. Personally I prefer the short version however I also like to follow the standard set by SharePoint. The only real limitation of the short version of the Content type ID is that you can only inherit 254 times from a base content type.

The main thing to consider there is that when the next release of SharePoint comes out will there be any duplicate Id in use in your solution. Therefore to avoid duplicate IDs use a long GUID when you create base content type which extends an out of the box content type (such as Item or Document). Then when you extend your base content type you can use 01, 02, 03 etc.

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