How to build a simple data comparison rule? I am working on Drupal 8.7.10 and I have installed a fresh Rules 8.x-3.0-alpha5.

Could someone help me to start a very simple rule? I am still struggling with the "new" Rules.

All I need is to send an email to a specific role when a specific field has changed.

"Rulesly" speaking, after a node has been updated, if the unchanged field value is different from the changed field value, then send an email to the manager, with a link to the updated node.

P.S. : I have also started to work with Business Rules so if someone can build this with this module, that sounds good too!
OK, I would then recommend Business Rules to build that kind of data comparison rule.

1- Create a first Variable "Value from Entity field" to store the original value of your field.

2- Create a second Variable "Value from Entity field" to store the current value of your field (i.e. the saved value after updating node)

3- Create an Action "Send an email"

4- Create a Condition "Variable data comparison" to compare the two variables you built in 1 and 2. Available variables are listed, use {{your_variable_name}}. Use the right operator, here I needed != (different from).

5- Inside your Condition, add the Action "Send an email" (in section "Items to execute if condition succeed")

6- Create a Rule based on an event "After updating Entity", define the context (entity type and bundle, here content and content type) and insert the Condition you built in 4. You should see a nice flowchart with your Rule label (white), then your Condition label (blue) and if YES, your Action label (green).

That's a start.

Hope this helps.