Some collets can be used on different machines, so for the ER system I use an R-8 ER-40 holder on the mill and a MT3 ER-40 collet holder on the tailstock of the mill, they can also be used with an ER-40 chuck mounted to the spindle. The MT3 holder was for a drawbar (threaded), but I machined a threaded tang so it would not turn in the tailstock. They sell other sizes of MT holders for ER40 chucks, so it could be used in a rotary table with a MT2 ER-40 holder.There are 5C and ER collet blocks in square and hex, very helpful for machining flats on round stock.
Never have seen an R-8 collet used on the lathe. Only seen 5C for the lathe. On the mill, especially if you do not have a power draw bar, an ER system may be easier to switch collets. I have a nice set of R-8 collets and almost never use them.