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What are the differences between variables HMXL and HBLT?

POP2 outputs two variables related to mixed layer dynamics: HMXL and HBLT. According to documentation 10. Model diagnostics and output — popdoc documentation they are
  • “HMXL and HMXL_2”, “cm”, “Mixed-Layer Depth”
  • “HBLT”, “cm”, “Boundary-Layer Depth”
The ratio of their values is very different (means are 61.05 and 39.74), especially in the tropics. The following two pictures are global maps of HMXL and HBLT provided on CESM LE diagnostic site.
I think POP2 SOM forcing files provided uses HBLT as the mixed layer depth. Can anyone elaborate on some of the physical differences? In particular, which of these variables is the thermally well-mixed layer?