It becomes important to understand where a lot of this bloodline stuff came from.
That was based on the 2nd ed game (not the PC game). BR (Birthright) was a setting that TSR (now Wizards of the Coast) had creted - Forgotten Realms, Planescape, Ravenloft, Dark Sun were all settings in 2nd ed.
Each one had things that set it apart from the others. The crux of BR was that the old gods died and the surving mortals on the battlefield inherited a portion of their divinity when their blood spilt on them.
This granted the ability to rule (regency) and extra abilities (blood abilities) that were in a way related to the strength of the bloodline. The bloodline was a means of measuring this divinity.
Bloodlines have three primary characteristics. A scion’s bloodline derivation (Anduiras (And), Azrai (Az), Basaïa (Ba), Brenna (Bre), Masela (Ma), Reynir (Re), or Vorynn (Vor)) specifies the old god whose divine power flows in the veins of his family. A scion’s bloodline strength (minor, major, great, or true) describes the purity of the scion’s family’s bloodline. A scion’s bloodline score describes the power of a specific scion’s divine essence. The manifestations of a bloodline are known as blood abilities.
The three parts of the bloodline are:
derivation - basically from which of the old gods this power came from.
Strength - the purity of the bloodline. That is how close to the original deity the character is or how diluted the bloodline has become over the generations.
Score - the numerical measure of the power.
In 2nd ed characters could be either blooded or non-blooded. Non-blooded got a 10% exp bonus. That type of game mechanic doesn’t really exist in 3.5. In addition a bloded character could be a regent (ruler) or not. A regent gained several benefits - 10 bonus hit points, a starting magic item and bodyguards (usually).
In 2nd ed there was only a superficial correlation between the score and strength of the blodline. In fact it was possible for cahracter with a tainted bloodline (very diluted) to have a higher score than a characte with a great bloodline. It can still happen but the present mechanics make it more difficult.
The use of scion classes was a way to help handle the exp bonus that non-blooded characters got in 2nd ed and also to provide a means of measuring the difference in power levels of the characters (that is roughly CR).
I hope that helps to explain some of the history and such. It is really difficult to use a computer game as the basis for understanding a setting - computer games are far too limited by software to adequately handle the complexities fo a RPG.