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From the press release:
Version 10 adds support for showing information in the menu bar including dynamic icon and text, with dynamic menus of macros that can be triggered easily.
A new Paste by Name facility allows you to quickly paste text straight from your clipboard history. The more comprehensive Clipboard History window has also been enhanced to include more information like word and line count, and additional options for setting the clipboard or pasting plain text.
Version 10 has a new action that allows you to select an area on the screen, and combining that with the OCR facilities allows you to OCR any area on the screen.
The editor is enhanced with the ability to save favorite actions, preconfigured with your choice of values, colors, and other settings. There is also a new Select Macro by Name command to quickly change to a specified macro, search fields in long insertion menus, control over evaluating condition results in real time, a menu item to select the last failed action, easy ways to go to a macro that calls the current macro and much more.
This version adds the concept of Subroutines, allowing you to pass multiple parameters to a macro to perform tasks shared by multiple macros and a Return action to return information back to the calling macro. There is also a new Try/Catch action to allow more complex error handling.
The Prompt for User Input action has added a bunch of new field types, like color and date selection.
And you can now export a macro as a Text Service or a Finder Quick Action to have the macro operate on selected text or the selected files.
The debugger now lets you jump straight to the action in question, and shows the value of recently accessed variables.
And so much more. More triggers, like triggering when you Unlock, when the Power Status changes, when the Appearance changes, or when you long press a hot key. Many more actions, like displaying a progress bar, create a unique file, and pause until a system change happens. Many more filters, like sort, reverse or shuffle lines, escape for regular expression, and quote for JSON. A plethora of new AppleScript facilities, and a truck load of other small changes to improve Keyboard Maestro across the board.
See the wiki What’s New section for a complete list of changes. With over a hundred other improvements, there is bound to be something for everyone.