Both random and fraction transfer STV have a number of variations.
There are a variety of methods for computing the winning threshold. I prefer to define them in words rather than through formulas.
Within these two types there are two options. A threshold can be static or dynamic. A static threshold is determined once at the beginning and is the same until the end. A dynamic threshold is recomputed every round and decreases as the number of exhausted votes increases. The threshold can also be a whole number or a fraction. Thus, there eight variations for determining the threshold.
The above descriptions of the Droop and Hare thresholds is an ideal which is not always perfectly realized. How well a threshold conforms to the ideal is how "tight" the threshold is. For example, a fractional threshold is slightly tighter than a whole threshold since there is greater precision.
If voters are required to rank all the candidates, then there will be no exhausted votes and the static and dynamic thresholds will be identical. If voters are not required to rank all the candidates, then there will be exhausted votes and the dynamic threshold will be tighter than the static threshold.
The ERS97 rules implement a threshold that is a variation of the Droop threshold. For a static threshold, Droop and ERS97 are identical. For a dynamic threshold, ERS97 is tighter than Droop. For a given candidate, his surplus votes are transferred only once. Thus, when the threshold decreases, these votes that are above the new threshold do not help elect any candidate. The ERS97 threshold takes into account these nontransferable surplus votes when computing the threshold and this results in a threshold that is smaller (tighter) than the Droop threshold.
The basic algorithm above requires that all surplus votes be transferred before the last place candidate is eliminated. However, if the last place candidate can't win, even if he received all the surplus votes, then he can safely be eliminated before transferring the surplus votes. Thus, we can delay the transfer of surplus until after this losing candidate is eliminated.
There are two reasons for doing this. First, eliminating last place candidates is much simpler than transferring surplus votes. Thus, we should delay transferring surplus as long as we can. Second, it will help minimize the number of times that a given vote will be transferred. If we transfer surplus immediately, then part of it could transfer to a candidate who will be eliminated in the next round. These votes would be transferred twice. If we delay the transfer of surplus until after this candidate is eliminated, then the same vote would be transferred just once.
The basic structure transfers surplus votes from only one candidate at a tie. One could instead transfer all surplus votes simultaneously.
One could eliminate candidates in batches rather than one by one. There are two rationales for this. The first is practicality. If there are many candidates, each with very few votes, then it would be tedious to eliminate them one by one. The second is efficiency. If it is known that several candidates can't win, then it makes sense to eliminate them all right away.
The first method is to eliminate all candidates who have fewer votes than a flat cutoff. This would take place immediately after all surplus votes have been transferred to allow all candidates to take advantage of surplus votes to avoid being eliminated in this fashion. This will be useful when there are many candidates with very few (or even no) first place votes. With many such candidates, it would take many rounds to eliminate them one by one. The batch elimination simplifies the count by eliminating all candidates with very few votes in one round. Note that this will possibly eliminate candidates who have a chance of winning. However, this is not a problem as long as the cutoff is chosen wisely.
The second method is the same as the first, except that the cutoff is chosen as a percentage, say 0.5%, of the total number of votes cast. This method is suggested in the STV rules in Behind the Ballot Box.
The third method is to eliminate all candidates immediately who have no chance of winning. This can happen before surplus votes are transferred and at any time throughout the count. To test if a given candidate can win, it is assumed that she will receive all surplus votes and all transfers from eliminated candidates. Under this assumption, a candidate can win if it is possible that she can beat all candidates between her and the last seat. If it is known that a candidate can't win then she should be eliminated immediately. It is possible that at a given round, that several candidates can't possibly win and they should all be eliminated in a batch.