Changing the values of dropdown dynamically based on conditions in Angular

Usual way

To do this you can add a change event emitter to know when you change your value

<mat-select (selectionChange)="changeMethod()" placeholder="Type" [(ngModel)]="Nm">

Then in your ts file

changeMethod(){
  if(this.Nm == 'label'){
    this.readFl = 'Y'
  }
}

And to disable your other selection you can either use an other variable as boolean and change it also in your changeMethod or add this to your select <mat-select placeholder="Read" [(ngModel)] = "readFl" [disabled]="Nm == label">. With Nm == label the select will automatically set disabled to true if Nm == label.

Ternary operator

You can use the ternary operator that act like an If/Else statement, to check if you are on the label select directly on your template.

<mat-select placeholder="Read" [disabled]="Nm == 'label'" 
                               [(ngModel)]="Nm == 'label' ? 'Y' : readFl">

By doing this you will do the following

if(Nm == 'label'){
  return 'Y'
else
  return readFl

This allow you to not add anything to your ts file, but doing this way, you will not store the 'Y' in your readFl variable, so if you need the value then with a submit or something like that, you will have to do something like before, and checking if your Nm is 'label'.

So finally the first method is better if you need the value 'Y' or 'N' after.