Power Tube Matching

Can someone translate the two #’s on the tube boxes?

What does the first two digit # signify? Plate current I think. And I assume millivolts, milliamps? Or what?

And the 2nd four digit # means transconductance I gather. What is transconductance?

How far apart can these #’s be before they are considered unmatched? And if you explain with percentage, I will need to know how high the numbers go, so maybe stick to actual number increases. My point being, on the internet people just say "5% difference." This means nothing if the plate current numbers can go up to 35 million, or only 100, or maybe 50, or 120.

So, if I Bob has 3 tubes that all have the first number of 14.3.....what numbers on a forth tube would be considered matched? 17.1, 18.4, 19.0, 25.8, or even 12.3?

What happens really if Bob used an unmatched pair of tubes in an amp. And it would be nice to hear about direct experience.

Thank you for your time,

TD