Does the True DSO metric consider all invoices or only open invoices

As per Oracle description for True DSO below,the highlighted line mentions about Open invoices but the function get_true_dso_ocm in code IEX_COLL_IND from file iexvmtib.pls calculates the value for all invoices.

True DSO - "The accurate and actual number of days credit sales are unpaid. This is a complicated formula as you have to tie every invoice back to net sales for the month in which the invoice originated.

Formula: True DSO per invoice = Number of days from invoice date to reporting date * (invoice amount / net credit sales for the month in which sale occurred)