Apportionment statutes

What does Apportionment statutes mean in American Law?

The definition of Apportionment statutes in the law of the United States, as defined by the lexicographer Arthur Leff in his legal dictionary is:

The term might be used to refer to any statutes governing any apportionment, but it primarily refers to those almost universal statutes which provide that estate taxes payable out of a decedent’s estate shall be apportioned to the beneficiaries of the estate according to a particular formula (usually pro rata), unless the testator shall have provided otherwise in his will.