The maximum saturation point of a pixel's photodiode in a digital sensor is the point at which it can no longer generate new electrons for additional photon strikes. At an electronic level, the saturation point varies with each type of sensor, as the maximum electron capacity of a given photodiode is dependent upon the physical area of the diode, its efficiency, its tendency to leak electrons, etc. Dynamic range can be described in a variety of ways, however a common way to describe it (often used by camera review sites like DXOMark) is as the ratio between maximum saturation, and the standard deviation of the sensors electronic noise floor.

The ISO 15739 specification you are referring to is describing a specific, standardized approach to describing dynamic range. From what I can find, this standard is a little convoluted, and used terms that don't seem to have an adequate explanation of why they are derived the way they are derived. The saturation point they are referring to, if I understand correctly, is actually 100/140th (71%) of the sensors actual maximum saturation level (as described above in my answer.) The full ISO 15739 specification is not freely available as far as I can tell, so the information I have to go off of is simply derivative works based on the ISO 15739 standard.

If my understanding is correct, however, dynamic range in this standard is described as the ratio between the point where shadow SNR is 1.0 and 71% of the sensors maximum saturation point (where no more electrons can be held in the photodiodes of each pixel.) The saturation luminance is that 71% of maximum saturation. In more detailed terms, ISO 15739 DR is the ratio between a luminance level that matches the noise ratio (say 8 electrons worth) and the 71% of the maximum saturation point, which for say Canon's 18mp sensors would be about 22000 electrons * .71, or 15620 electrons. That is a ratio of 1952.5:1 in terms of electrons, or in terms of stops, a little less than 11 (2^11 = 2048).