There is a description of the constant
IntlCalendar :: FIELD_MILLISECONDS_IN_DAY
Calendar field encompassing the information in IntlCalendar :: FIELD_HOUR_OF_DAY, IntlCalendar :: FIELD_MINUTE, IntlCalendar :: FIELD_SECOND and IntlCalendar :: FIELD_MILLISECOND. Range is from the 0 to 24 * 3600 * 1000 - 1. It is not the same time as the DST.
It seems to be clear that we are talking about the number of milliseconds elapsed since the date change, but the last paragraph says that this is not the case. That's about the "discontinuities" and especially about "the wall time" Vasche did not drive.