Let me go back to the original question of what all of this means.
Like whaler said, the numbers are designators for the tip opening, that is the distance of the inner tip surface of the reed to the inner surface of the tip rail. There is a certain consistency between all manufacturers but there are also deviations. A pretty complete
listing of the different numbers is in this chart.
When you look at the chart, you will also see that some manufacturers are not using numerical values but letters, like in the case of Selmer.
The significance of the tip opening is that the wider the gap is, the more air will you need to push through to make the reed vibrate and get a sound out of your instrument. Also, you will need more pressure on the reed to control it so it is not necessarily a good thing to have a big opening. On the down side, an smaller tip opening will be easier to play but not give you the power (for what it's worth unless you are deaf or play in a Rock'n Roll band).
Tip openings are also deceiving, a mouthpiece with a high baffle will play much easier with a "nominally" big opening than one with a low baffle but we can revisit this topic 6 months from now.
Next is the chamber, that is the part of the mouthpiece with the biggest volume and essentially is the entire space inside the beak portion of the mouthpiece. Some are large chamber (typically a more mellow or deeper sound), some are medium (balanced - one size fits all) or small chamber. The smaller the chamber, the brighter the sound but it also introduces more of a challenge to hit the notes correctly, whereas large chamber mouthpieces are more forgiving but also harder to correct (bend up or down) if the instrument is out of tune, which is a feature of saxophones, not a problem.
Larger tip openings can be compensated for using softer reeds and vice versa and it all comes down to the specific combination of player, reed, mouthpiece, instrument and desired kind of music what will work. Change one parameter and you will have to change at least one or two of the others to find the sweet spot again.
Hope this helps!