Wondering about frame numbers with fractional digits during render like "1.15 2.15 3.15" instead of "1 2 3"

[BeneZ]

Hi,
I see sometimes frame numbers with fractional digits during rendering.
What does it mean?
Has it to do with motion blur maybe?

Thanks for explanation

[hermit.crab]

These medial frames are utilized for interpolating an object's coordinates for motion blur or other functionalities that require such calculations (fluid simulation for example). The timeline will go to these frames automatically, however, the rendered frame will be marked with a whole number. And yes, V-Ray's motion blur utilizes these medial frames.