Ex: Rendering

You can render a view by Preset: Draft, Low, Medium, High, Presentation

Render tab > Render panel > Low

Click Render

Low anti-aliasing and a raytracing depth of 3 reflections / refractions are processed

Render tab > Render panel > Render Region

Set the higher render preset and destination

Render Preset: Presentation
Destination: Viewport

Very high anti-aliasing and a raytracing depth of 9 reflections / refractions are processed