A core reason for needing side scatter light scatter calibration is to normalize the variability in flow cytometer side scatter collection angles. One of the biggest factors leading to variability in flow cytometer collection angles is the flow cell and the numerical aperture of the collection lens, shown below.
The limiting system collection angle from these components can either be due to the internal flow cell dimensions, the external flow cell dimensions, or the collection lens numerical aperture. While some systems are limited by their internal flow cell dimensions, most system are limited by their external flow cell dimensions. These can be seen in the instrument preset examples below.