Symbols that need precise musical start times such as notes, tuplets, and clefs, use the ruler when they are entered in the score. The ruler provides a way of specifying which beat or sub-beat in the measure the symbol is to be entered. The ruler is divided into a series of different sized marks that correspond to strong beats (the longest), weak beats (slightly shorter), and sub-beats (the shortest). Below are some examples different rulers over different meters.
The music ruler allows you to enter notes anywhere in the measure without first having to enter rests. By default, the ruler is at a resolution of 16th notes allowing you to enter a note on the "1", "e", "and", or "a" of the beat. You can globally change the resolution of all of the measures, or you can just change the current measure's resolution by entering a note with a value less than a 16th note.
As you move the cursor across a measure, a dotted vertical line (in the selection color) snaps to the beats and sub-beats of the measure, and the current beat and tick is displayed in the status bar. Clicking the mouse will enter the symbol on the beat or sub-beat where the vertical line is. If the measure is without notes, and you are entering notes, you can enter on any beat or sub-beat of the measure. However, if there are notes in the measure, the cursor will not allow you to enter a note that starts within the duration of a previous note. See below for some examples: