The ERS97 rules are a combination of the principles behind random transfer and fractional transfer STV. In my opinion, the rules are excessively and unnecessarily complicated. See http://www.electoral-reform.org.uk/votingsystems/stvrules.htm for the official rules.

The ERS97 rules are independent of the order of the ballots. The first time a candidate's surplus votes are transferred, the method described in Section V is used. These transfers can lead to secondary surpluses. However, the secondary transfers of surplus votes are done differently. For secondary transfers, only the last group of ballots received will be transferred. This resembles how ballots are transferred with random transfer STV methods. Tertiary transfers are possible, but much less likely to occur.

The ERS97 rules differ from the other rules presented here in many ways. See the python code for more details.