I believe it's probably refering to "pre-drilled" hole that you make when trial fitting the sender. After the big hole is drilled set the sender in and use it as a template for drilling the screw holes.

On the tank I used on one of my projects it used the screws with drill points that self tapped into the tank top.

Just a tip when you get all the holes drilled before you you install the sender for the final time. Wrap the end of a piece of wire/coat hanger with some masking tape after a few wraps twist the tape and give it a few more wraps sticky side out and use it to fish out the shavings that have ended up in the bottom of the tank.