Rule

Also known as Activation Rules.
Outlook activates some types of add-ins if the message or appointment that the user is reading or composing satisfies a specific activation rule.
This is true for all add-ins that use the 1.1 manifest schema and for custom pane addins.
The user can then choose the add-in from the Outlook UI to start it for the current item.

To have Outlook activate an add-in for specific conditions, specify activation rules in the add-in manifest. There are two Rule elements that you can use:
Rule element (MailApp complexType) that specifies an individual rule
Rule element (RuleCollection complexType) that combines multiple rules using logical operations

So when you specify an individual rule in a manifest, you must use the xsi:type attribute

Mode Or / And

Note: The FormType attribute applies to activation rules in the manifest v1.1 but is not defined in VersionOverrides v1.0. So it can't be used when ItemIs is used in the VersionOverrides node.

ItemIs

Available in: Read, Compose, Custom pane
The FormType attribute was added in manifest 1.1
Checks to see whether the current item is of the specified type (message or appointment). Can also check the item class and form type.and optionally, item message class.

<Rule xsi: type="RuleCollection" Mode="Or"> 
   <Rule xsi:type="ItemIs" ItemType="Message" FormType="Read" /> 
   <Rule xsi:type="ItemIs" ItemType="Message" FormType="Edit"/> 
   <Rule xsi:type="ItemIs" ItemType="Message" FormType="ReadorEdit" /> 

   <Rule xsi:type="ItemIs" ItemType="Appointment" FormType="Read"/> 
   <Rule xsi:type="ItemIs" ItemType="Appointment" FormType="Edit"/> 

   <Rule xsi:type="ItemIs" ItemClass="string" IncludeSubClasses="true"/> 
</Rule > 

ItemHasAttachment

Available in: Read, Custom pane
Checks to see whether the selected item contains an attachment.

ItemHasKnownEntity

Available in: Read, Custom pane
Checks to see whether the selected item contains one or more well-known entities.
Does the subject or body contain an email address

<Rule xsi: type="RuleCollection" Mode="Or"> 
   <Rule xsi:type="ItemHasKnownEntity" EntityType="Address"/> 
</Rule > 

ItemHasRegularExpressionMatch

Available in: Read, Custom pane
Checks to see whether the sender's email address, the subject, and/or the body of the selected item contains a match to a regular expression.

<Rule xsi: type="RuleCollection" Mode="Or"> 
   <Rule xsi:type="ItemHasRegularExpressionMatch" 
        RegExName="HasURL" 
        RegExValue="http://www.bettersolutions.com/?id=[a-zA-Z]{14}" 
        PropertyName="Body"/> 
</Rule > 

RuleCollection

Available in: Read, Compose, Custom pane
Combines a set of rules so that you can form more complex rules.