Action

ExecuteFunction

When the type attribute is set to ExecuteFunction the command will call the function specified by the FunctionName tag.
This function name must exist in the FunctionFile element.

ShowTaskpane

When the type attribute is set to ShowTaskpane the command will display the task pane specified by the SourceLocation tag.

<VersionOverrides xmlns:bt=""> 
   <Hosts> 
      <Host> 
         <DesktopFormFactor> 
            <ExtensionPoint> 
               <OfficeTab id="TabDefault"> 
                   <Group> 
                      <Control> 

<Action xsi:type="ExecuteFunction"> 
   <FunctionName>onCommandButtonClick</FunctionName> 
</Action> 

<Action xsi:type="ShowTaskpane"> 
   <SourceLocation resid="res_Urls_TaskpaneID"/> 
   <TaskpaneId>MyTaskpaneID</TaskpaneId> 
   <Title resid="res_ShortStrings_PaneTitleID"/> 
   <SupportsPinning>true</SupportsPinning> 
</Action> 

FunctionName - Required. Specifies the name of the function to execute.
The function is contained in the file specified in the FunctionFile element that is declared earlier in the manifest.
The function must exist in the global DOM namespace of the function file.

SourceLocation - Required. Specifies the source file location for this action.
The resid attribute must be set to an id in the Urls element of the Resources element.

TaskpaneId - Optional. Specifies the ID of the task pane container.
If there are multiple task panes then each task pane should have a unique task pane ID.
You should use the same ID for different actions that share the same task pane.
When a user chooses a command with the same task pane ID, the pane will remain open but the contents of the pane will be replaced with the corresponding SourceLocation.

Title - Optional. Specifies a custom title for the task pane action.
The resid attribute must be set to an id in the ShortStrings element of the Resources element.

SupportsPinning - Optional. (Added V1.1) Whether to pin the task pane so it stays open even when the selection changes.
Only supported in Outlook on Windows, build 7628.1000 or later.
Only supported in Outlook on Mac, build 16.13.503 or later.
Only available for Outlook Messages and Meeting Requests (not Appointments or Meetings).
Sample - github.com/OfficeDev/outlook-add-in-command-demo