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When the $KeepPrivate field is present in a document with a value of "1", you will be unable to forward, print or copy the document.
There are at least 4 ways in which the $KeepPrivate field (with a value of "1") can be added to a document. Please be advised that however you add this field, it is only a deterrent and not a true security feature. There are other ways people can still copy, forward or print the document if they are determined to do so.
Of the following options, only option 3 requires direct interaction from the end user of the form. Options 1, 2 and 4 do not require any interaction from the end user of the form, and may be the reason this field is added without the user/creator of the document's knowledge.
Option #1: Through Access Control List (ACL) Settings
If the creator of a document has author access or above in the ACL of a particular database, however the 'Replicate or copy documents' permission is not selected for that user, then whenever that user creates a document, the $KeepPrivate field will be created and its value will be set to "1". For more information on this option, please consult the Domino Administrator's Guide.
Option #2: Through the Design Client
Assuming you have the appropriate access in the ACL for the database, you can access and change the form properties in the Design client. On the Security Tab of the Form Properties, you can select 'Disable printing/forwarding/copying to clipboard'. Any document based on this form will have the $KeepPrivate field with its value set to "1". End users of the form may not realize this option is in place when they create documents using this form. For more information on this option, please consult the Domino Designer's Guide.
NOTE: Notes 3.x clients do not recognize the "Prevent copying" functionality. Therefore, Notes 3.x users are able to print, forward, and copy memos even when the "Prevent copying" option is enabled.
Option #3: As a 'Delivery Option' when sending mail
When you create a new message, you can select Actions, Delivery Options, and on the 'Basics' tab, you can select the 'Prevent copying' option and that will add the $KeepPrivate field (with a value of "1") to the sent message so that the receiver of the message cannot copy, print or forward the message with history. For more information on this option, please consult the Notes Online Help
Option #: 4: Manually adding the field to the form
You can manually add this field to a form if you have Design access to the form in question by either directly adding the field to the form, and setting the default value for the field to "1", or you can write an agent that will add the field, and set its value to any or all existing documents in the database. For more information on this option, please consult the Domino Designer's Guide.
