Any scope with a large zoom range for example 10x such as the March 2.5-25x52 needs to have a parallax adjustment to keep the image in focus and parallax free. Scopes with a 3x zoom such as 3-9x42mm generally don't have a parallax/focus knob. It is fully explained in detail in the article posted at the beginning of this section;
https://www.longrangehunting.com/threads/parallax-vs-focus.1988/