Key Settlement in
Term Rate refers to the time interval δ between the date T₀ when the rate r is agreed (i.e. fixed or reset) and the date T₁ when the rate's "spanning time interval" [T₁,T₂] starts, as described at Term Rate
It is expressed as an object of type
It also defines the DateBump and Calendar conventions used in the determination of both T₁ and T₂

Specifically, T₁ is constructed by shifting T₀ by δ
Subsequently, T₂ is constructed by shifting T₁ by the interval specified in
Tenor and then bumping the resulting date according to the conventions here.

Typically, it is defined as a
Single Period with its key Time Unit set to B so that it effectively defines the number of business days between T₀ and T₁
But there are notable exceptions, such as Libors of currencies other than EUR or GBP, where the entry here must be defined as an object of type
Period containing a chain of single periods
Such rates (for example the USD Libor) are settled by shifting the fixing date forward by N business days (N = 2) according to a certain calendar (UK), but then the produced date is further bumped according to some other calendar (joint UK + US).
These cases are then accommodated with a Period object of the composite type because then the settlement date is constructed by shifting the reference date first by the first Single Period and then by the second Single Period, whereby the second Single Period is set to contain zero days and the required calendar.

In the case of a composite Period entry, the end date is first constructed as described above using the conventions of the first Single Period
Then the resulting date is further shifted by the remaining Single Period objects in sequence.