[?] Derivative results with "1." and ".0" format?

Questions
1

I’m trying to figure out why

D[0.5*x1^2 , x1]

results with

1. x1

What does “1.” represent?
For some other expressions I obtained results containing “0.” which I can’t wrap my head around.
Sorry if the questions is stupid and it’s something simple.

2

it just means that the prefactor is ‘one’. But rather being the precise 1, it can also be an approximate (machine-precision) one. This is the case here. it is not exactly integer 1, but a floating point number 1. If you do:

D[(1/2)*x1^2 , x1]

you will see x1. no prefactor. The entire idea (apart from a few exception) of Wolfram Language is exact input = exact output. Machine-precision input = Machine-precision output. etc