XMOS chip's Part Marking scheme is:
That's the part marking for newer L-series chips.
1) what means "US", it means United States?
The SU-series chips have SK there. I don't find it likely that XMOS has their
chips manufactured in the USA and Slovakia. It probably does identify the
factory in some way though.
2) what means "MM", it is 2 digital month?
3) what means "YY", it is 2 digital year?
It does seem the MMYY part identifies a date, but it seems to be in YYWW
format (year and week of the year).
4) what is LLLLLL.LL, the lot code.
It's a number (well, alphanumerical actually) that identifies the batch of
chips this belongs to. The trailing LL seems to be 00 usually.