Indicates the origin of the collection, given the row's subtree. The
architecture implementation for statistics collection imposes that rows
with distinct usdAcctngSelectionSubtreeType may not be collected into the
same accounting file. Conversely, different files (configured from
acctngSelectionFile) entries are necessary in order to collect from
subtrees (configured from acctngSelectionSubtree) that imply into
different subtree types (usdAcctngSelectionSubtreeType).