When will my Redshift subscription be used vs. the included Redshift CPU?

If you have GPU Devices or Hybrid Rendering enabled in the Redshift Preferences and an available Redshift license, the Redshift license will be used. The GPU (and CPU in the case of Hybrid Rendering) will be used to render the project.

If no compatible GPU Devices are enabled in the Redshift Preferences and you have an available Redshift CPU license, the Redshift CPU license will be used. Only the CPU will be used to render the project.

If no compatible GPU Devices are enabled in the Redshift Preferences and you have an available Redshift GPU license but no Redshift CPU license, the Redshift GPU license will be used. 

If compatible GPU Devices are enabled but no available Redshift license is found, an available Redshift CPU license will be used to render the project using only the CPU.

Redshift licenses are consumed when you begin a render (either via the IPR, Render View or DCC-specific render command). Any change to the enabled devices will be reflected with your next render (if you disable all GPUs the Redshift subscription will be freed the next time you render).