HSC is obvious master clock
LSI in a low speed (Independent)RC clock just for watchdog timer 40kHz
LSC is for a real-time-clock RTC.
There are hundreds of passive resonator options for MEMS or Crystals (XTAL) case, load capacitance,
initial tolerance at room temp and
stability over specified temp range. Since the cut on these resonators follows a downward parabolic curve the + error is small at room temp or perhaps wrist temp and large -ve error above and below room temp when you search for in stock [x] "32,768 kHz", whjch is a binary multiple of 1 Hz.
Adjustable cap. may be used to null initial tolerance errors near room temp or as desired. The "load cap" value is the series combination of two caps.
But for those who want better temperature compensated time of day, one would choose from
or if you want even better +/-3 ppm XO or Xtal Oscillator and just drive the input port. Looser specs for cheaper parts exist too.
I remember hiring an engineer from Philadelphia when I worked in RB, San Diego, Cali, who was a recent graduate and did not know what a crystal was.
but he was a good test engineer and adapted to programming automated tests. Whereas I as Test Eng Mgr had to solve a critical field failure after boards were farmed out to a board shop for this new factory that were all failing in the field. So I took a grinder to this encapsulated crystal oscillator with a film tuning lid and found rust. Then I went to the QA guy to call the Fab shop and told them to wash the PCBA flux before putting on the Xtal. Elapsed time 2 hrs.
Problem solved.