Each row has a solid top line, a dashed middle line at the x-height and a solid baseline. The middle line is what makes letter proportion teachable: a letter that overshoots it is wrong in a way a child can see for themselves.
Line height is the choice that matters most. Toddler sheets print letters close to an inch tall, because a child without fine motor control writes from the shoulder and needs the room. Elementary sheets are near ruled-paper size and fit six repetitions to a line.
Letterforms are embedded in every file as outlines rather than as text in a font, so a worksheet prints identically everywhere and does not depend on anything being installed.