Hello, may I ask is there a way to make STM show first N characters while hiding the rest?

In my case, all the character has to be in the STM from the beginning, so I cannot use append to add the hidden characters later.

Also as the number "N" changes from frame to frame, I also can't use <delay> or <c> to achieve the effect...

Are there any ways to achieve the desired effect? Thank you for your help.

I think <pause> would be your best bet! You can call Continue() in your code to have STM continue reading past this point.