It corresponds to the reference period taken into account to calculate the distribution coefficients.
In effect, the principle of these types of distribution is to distribute to the other dimensions the balance (in amount) or the quantity (in Non-financial Unit) of each dimension to be distributed in proportion to the postings of said other dimensions to a so-called distribution nature.
The "Period for calculation" is used to only keep the postings for a given period and thus to calculate the distribution coefficients related to said period.