Musical positions and lengths, e.g. of notes in a sequence, are formatted as Bars.Beats.Ticks.
For example, position 17.3.8000 means the 17th bar, the 3rd beat within that bar, 8000 ticks from the start from that beat.
A length of 0.2.6000 means a length of 2 beats and 6000 ticks.
There are 12000 ticks per beat (aka quarter-note). A 16th is 1/4th of a beat so it's 3000 ticks.
If you have a composition with tempo 120 BPM, then every tick corresponds to 0.04166667 ms.
The time signature is defined by the composition and can be edited via the composition's context menu -> Properties. By default, there are 4 beats in a bar.
You can edit a position or length by dragging the values up/down, by using the mouse wheel, by clicking the previous/next grid position buttons, or by double-clicking the value to enter a numeric value.
In case of double-clicking a musical position then the format is even more precise: Bars.Beats.Ticks.MicroTicks. Why is that?
Starting with MuLab version 8 events can play audio and therefore it may be necessary to have sample-accurate control over event positions.
And then the 12000 ticks per beat resolution is not fine enough because eg. using a 48 kHz samplerate and tempo 80 bpm then a single tick spans 3 samples. That's not sample-accurate.
So starting with M8 each tick has 384 microticks. So the very finest resolution is 4608000 microticks per beat. That's still 24x finer than using a 192 kHz samplerate at 60 bpm.
Now as it would be user-unfriendly to always display musical positions as Bars.Beats.Ticks.MicroTicks, musical positions are displayed as Bars.Beats.Ticks but when you double-click such position value, then the most detailed format is used.
You will only need it in rather exceptional cases, maybe you won't need this high precision at all. But it's there if you need it.
When entering a new value you don't need to type the complete Bars.Beats.Ticks.MicroTicks format, your input is interpreted left to right.
So when you type 45 it will be interpreted as bar 45.
When you type 45.3 it's bar 45, 3rd beat.
When you type 45.3.6000 it's bar 45, 3rd beat, 6000 ticks.
When you type 45.3.6000.278 it's bar 45, 3rd beat, 6000 ticks, 278 microticks.