Microsoft has recently added a Table component to Power Apps called Modern Commands. These Modern Commands are meant to replace the Classic Commands that were commonly known as the Ribbon or Ribbon Buttons. A user-friendly interface was added to create, modify, and organize these commands that can be accessed through the Modern App Designer. Something new that Modern Commands offers us is the ability to use JavaScript OR Power Fx for the actions attached to them.  Source

Where to Begin 

To use Commands in a Model-Driven App, it must be an app that is set up to use the Modern App Designer. To do this, simply Open your target solution, select +New > App > Model-Driven App, and then select Modern app designer in the pop-up window. Click Create, and then select a name for your app. You will be dropped onto the main page of the Modern App Designer. From here, click +Add Page, select Table-based view and form, and choose your table whose Command Bar you want to edit. Once added to the app, use the ellipses next to the table name and select Edit command bar. Source

Next, choose which Command Bar you want to edit 

• Main Grid – Viewable from main views 
• Main Form – Viewable from main forms 
• Subgrid view – Viewable on embedded subgrids 
• Associated view – Viewable from related data views, such as the Related dropdown tab on another table’s main form 

Once you select an option, you will see a list of default commands for the selected table and Command Bar. You can switch Command Bars at any time by using the Switch command bar dropdown in the top right.  

Selecting the +New button will give you 4 options: 

• Command – A basic solo Command button on the Bar 
• Dropdown – A dropdown menu for other Commands 
• Split Button – Similar to a Dropdown, but the label portion represents a Command 
• Group – An organizing label for Dropdowns and Split Buttons 

The “Command” menu on the right will allow you to edit the Command’s label, icon, action, visibility and other info. For the action portion, you can select JavaScript or Power Fx. The first time you select Power Fx the system will create a component library. Doing so will not disable JavaScript. 

One of the major changes from the old Ribbon is that the visibility rules for Commands can only be written in Power Fx.  Source

Solution Management of Commands 

Commands are a component of Tables, and they are integrated so that Power Apps will record any dependencies that are used in your command. To add them to your solutions, you will select the Table that your command is attached to and use the Add Existing button, and then navigate to the Commands tab to find any custom commands written for that table. Source

Commands Limitations 

• Apps that do not run on the Modern App Designer do not support Commands 
• The Command designer is not backwards compatible with Classic Commands written with the Ribbon Workbench or other tools 
• Not all Power Fx functions are supported 
• Dataverse is the only supported connector for Power Fx 
• Commands cannot be transferred between apps 
• Visibility rules are currently limited 
• The process to delete commands is currently very tedious 
• Buttons cannot be dynamically populated 

 Source