Additive synthesis could be considered the reverse approach to subtractive synthesis. See Sound basics overview, Tones, overtones, harmonics, and partials, and How subtractive synthesizers work.
To obtain an insight into the additive synthesis approach, consider the fact that all sounds are a sum of various sine tones and harmonics.
In additive synthesis, you start out with nothing and build a sound by combining multiple sine waves of differing levels and frequencies. As more sine waves are combined, they begin to generate additional harmonics. In most additive synthesizers, each set of sine waves is viewed and used much like an oscillator.
Depending on the sophistication of the additive synthesizer you are using, you will either have individual envelope control over each sine wave or you will be limited to envelope control over groups of sine waves—one envelope per sound and its harmonics, or all odd or all even harmonics, for example.
Logic Pro doesn’t provide a true additive synthesizer, but aspects of the additive synthesis approach are used in Vintage B3 and all other drawbar organs. In Vintage B3 you start with a basic tone and add harmonics to it, to build up a richer sound. The level relationships between the fundamental tone and each harmonic are determined by how far you pull each drawbar out. As Vintage B3 doesn’t provide envelope control over each harmonic, it is limited to organ emulations.