Hi Nada, 

You can use enitity level validation to give mulitiple validation on single attribute or you can configure it in microflow to check its empty and equal.

Refer this images below

There are several ways to do this. You could:

1. add an on change / leave microflow that checks the value as the user is still working on the page
2. add a validation flow to the save / proceed button in your form. 
3. Use custom validation and write an expression that both checks whether the field is empty, AND checks whatever else you need to check

For the first two options, your validation flow will look something like this, with a validation per split:

Validation is also covered in the Crash Course:

https://academy.mendix.com/link/module/324/lecture/2934/7.4.2-Adding-Validation-Rules

So you can explore it at your own pace

I don’t know((