On HashKey Chain, fan-in means many addresses sending HSK to one account and fan-out means one account paying many. The address format hides both, so the pattern comes from counting transfers.
Counting senders and receivers
Count the distinct senders that feed one address, then count the receivers it pays onward. Short gaps between inbound and outbound transfers matter most. Remember that every transaction carries two cost parts, one on the Layer 2 and one for posting data to Ethereum mainnet, so fees alone say little about intent.
Phrasing the pattern
Describe pooling followed by redistribution and attach the hops you resolved. Leave out the word mixer unless your evidence names a service.