Since BABE is probabilistic this is the average expected block time that
we are targetting. Blocks will be produced at a minimum duration defined
by SLOT_DURATION, but some slots will not be allocated to any
authority and hence no block will be produced. We expect to have this
block time on average following the defined slot duration and the value
of c configured for BABE (where 1 - c represents the probability of
a slot being empty).
This value is only used indirectly to define the unit constants below
that are expressed in blocks. The rest of the code should use
SLOT_DURATION instead (like the Timestamp pallet for calculating the
minimum period).