I figured the easiest way to explain this is to describe the process of unlocking skills.
When you first buy a 'Mech (or are otherwise just starting the skill unlocking process), you pick a role for your 'Mech:
![Posted Image](http://i.imgur.com/Yz3lJ0c.png)
The numbers represent the maximum "tier" that can be unlocked for that specific role, with each role being ranked from 1 to 5. (A Scout needs to be the most mobile, Brawler less so, Support least of all. Explanations will be provided later.)
Scouts do just that - scout. They provide information for the rest of the team and excel at flanking maneuvers and harassing.
Snipers fire from long ranges with ERLL, Gauss, PPC, etc.
Brawlers get in close and kick the snot out of the other 'Mechs.
Supports provide LRM cover (BOO! I know...) and other support (ECM, AMS, possibly arty/airstrike benefits?)
Skirmishers fight at medium ranges and rely on outmaneuvering rather than brawling; they tend to stick with heavier 'Mechs and are 'Jack-of-all-Trades'.
Once you select a role, you go to each separate sub-category: Mobility, Survivability, Weapons, Operations, and Auxiliary. These are shown below:
![Posted Image](http://i.imgur.com/0W0D13c.png)
(These would all connect to form a tree, but it's too wide to show on the screen...)
![Posted Image](http://i.imgur.com/2i9dxQi.png)
5 Scouts need to be most mobile, obviously.
4 Skirmishers need to be able to flank and stick with any Assault/Heavies they are escorting.
3 Brawlers need to be able to spread damage when brawling.
2 Snipers have time to react to threats, so don't need as much mobility.
1 Supports just have to stick with the deathball...
![Posted Image](http://i.imgur.com/5SWvwyT.png)
5 Brawlers need to be able to take a beating.
4 Skirmishers need to be able to trade without worrying too much about their fragility.
3 Scouts need to be able to escape from behind enemy lines without it being a death sentence if they're caught.
2 Supports need to be able to survive flanking attacks.
1 Snipers shouldn't be receiving too much return fire.
![Posted Image](http://i.imgur.com/QLBqPFy.png)
This one's a little different. Each role tends to focus more on one type of weapon quirk than the others. Longer range fighters get range quirks, while roles that require quick take-downs get cooldown quirks.
![Posted Image](http://i.imgur.com/cqUaPOC.png)
5 Support 'Mechs don't rely on speed, strength, or weapon power to win - they rely on their team, and need to keep providing cover for them.
4 Snipers need to know which targets have priority.
3 Scouts need to be able to relay information to their team quickly and accurately.
2-1 Skirmishers and Brawlers are more focused on other strengths (armor, weapon quirks, etc.)
![Posted Image](http://i.imgur.com/6WpYSMg.png)
5 Again, Support 'Mechs focus on helping the team.
4 Brawlers would benefit from Radar Derp, AMS, and Seismic.
3 Snipers need Advanced Zoom? (*cough* this is just preliminary *cough*)
2 Meh...
1 This could use some work...
Anyway, hopefully you get the idea.
Depending on your role, you will be able to unlock up to a certain tier for each specific skill tree. For example:
A Scout can unlock 5 nodes on each of the Mobility skill tree branches (Accel/Deccel, Speed Tweak, JJ Thrust), while a Support can only unlock the first node on each. This means if each node provides a 3% increase for, say, Speed Tweak, the Scout can unlock up to 15%, while the support can unlock only 3%. This way, no skills are "locked" or blocked by, what some would call, useless skills.
In the weighted total you can see the maximum number of skill tree branches available. Some roles (Scouts, Supports) have more than others (Skirmishers, Brawlers). The max number of Skill Points could be below the minimum to make it fair. Let's say, 70. Then, underperforming 'Mechs could receive more, maybe up to 80, while OP 'Mechs would only get, say, 60. Also, because the number of Skill Points is less than the current number available, each node would provide more benefit when it is unlocked, for the same total when fully unlocked. Finally, the percentage increase for each node could also be affected by the weight class of the 'Mech, as is currently the case in the PTS (Lights get more benefit per node than Assaults).
Thanks for reading through all this, or at least looking at the pictures. Again, suggestions are welcome.