If other tickmarks are labeled, then you could do some detective work (of sorts) to figure out the unlabeled tickmarks.
For example, let's say we had a number line with 1,2,3,... and let's say that 7 was covered up or erased or smudged. So we have 1,2,3,4,5,6,__,8,9. We could then easily determine that 7 must go in that blank spot. This is just one example of course.
Another example could be that if we had a tickmark right in the middle of two whole numbers, say 0 and 1. This unlabeled tickmark would most likely be 1/2 = 0.5 as its at the halfway point between 0 and 1.