Hostfully currently supports 1 level of Master - Sub unit, meaning 1 Master unit and multiple Sub units. That's the traditional way where:
A booking to Master unit will block all Sub units
A booking to Sub unit will block Master unit, and leave other Subs open
There is another use case where another level of Sub-units are added. Let's say there are 3 properties in one compound with below setup:
A Master unit for 3 properties: A, B & C
Sub-units for each property: A, B, C
Groupings where guests can book 2 units at once: AB, AC & BC

That makes 7 properties in total:
Master ABC and 3 Subs A | B | C will have Master - Sub relations
Grouping A-B will consume iCal from Property A & Property B
Grouping A-C will consume iCal from Property A & Property C
Grouping B-C will consume iCal from Property B & Property C
With this setup, let's go through the scenarios below:
A booking / block for Master property ABC will block:
All 3 Sub-units A | B | C
Then, iCal blocks will be added to 3 Groupings AB, AC & BC
A booking / block for any Sub unit (let's say A) will block:
Master unit
Grouping AB & AC
A booking / block for any grouping property (let's say AB):
PM needs to manually block Sub unit A & B
Then, Master Unit will be blocked
And, Grouping BC & AC will be blocked
Note:
iCal sync is not instant, and a manual click on Sync button is necessary to push update.