You can use customization to match "your current frame and musculature."
= if you have strength and/or agility higher than 3 you can increase the cyberlimb to match this.
If you boosted your strength and/or agility beyond the natural maximum (with for example other cyberware, exceptional attribute quality or adept powers) you can still only customize your cyberlimb up to your racial natural maximum. Or as the rule book state: "If either of your limb’s attributes are increased beyond your natural maximum for that attribute, you can’t use the cyberlimb (so don’t overdo it), but you can still add cyberlimb enhancements."
Once you customized the cyberlimb up to your strength and/or agility you can still enhance the cyberlimb slightly more. This is called enhancing. You can only enhance a cyberlimb a maximum of up to +3 strength and/or agility and/or armor. All enhancements cost capacity. You can only get armor 2 at individual cyberlimbs at chargen (as armor 3 for a cyberlimb is availability 15).
Example 1;
So if you have a Troll with STR 10 and AGI 4 your cyberlimb will have STR 3 and AGI 3, but you can customize the cyberlimb up to STR 10 and AGI 4. It will, however, increase the availability of the cyberlimb by +8 (in this case you will be fine as base cyberlimbs have availability of 4 and 4+8 is not more than 12 so you can get it at chargen). It will also cost you 8x5,000=40,000¥
- After your cyberlimb have STR 10 and AGI 4 you can enhance it with up to +3 STR, +3 AGI and/or +2 (at chargen) armor. A STR 13, AGI 7, Armor 2 syntetic cyberarm would cost you 1 essence, 20,000¥ (limb) + 40,000¥ (custom) + 19,500¥ (str) +19,500¥ (agi) +6,000¥ (armor) = 105,000¥ and 8 capacity (which is all capacity a syntetic cyberarm have to offer).
Example 2;
If you have a human that dump stat STR and AGI that buy a cyberarm will have STR 3 and AGI 3. You cannot customize it higher. You can still enhance it with up to +3 STR, +3 AGI and +2 (at chargen) armor.
- A STR6, AGI6 and armor +2 syntetic cyberarm would cost 1 essence, 20,000¥ (limb) + 19,500¥ (str) +19,500¥ (agi) + 6,000¥ (armor) = 65,000¥ and 8 capacity (which is all capacity a syntetic cyberarm have to offer).