Client trading activity. Sometimes market type orders, sometimes stoploss stopentry or limit orders.
Flow traders / market makers at banks leverage this information in a few main ways in FX
1) Front running. Entirely legal in FX to front run client limit orders. There is also no centralised exchange so the criteria for large orders and when one should fill them are non standard.
2) Leaning on their trading books. I.e. knowing where their risks etc are positioned (three customers buying eurusd off them in quick succession - probably means something). Bunch of stoplosses sitting just below current levels - be careful etc etc etc
3) Information - if a certain customer is always directional (i.e. when they buy, the price tends to go up soon after) or always 'away' (i.e. they buy 200m eur from you, and the same probably from your mate down the road) or smart (i.e. you know and respect the people making their trading decisions) then follow their lead.
4) Pricing power - the more flow you see, the more times you will have a chance to match buyers and sellers, sitting in the middle making a few sponds for minimal (or sometimes even zero risk).
5) As a proxy - if your client is a good representative of a large cross section of the market, then you can make some more robust assumptions about what people are doing in the market. Certain types of client play to this scenario better than others.
Of course, not all traders at banks are flow traders. They have pure prop traders as well (often multi product), but flow trading is a big part of the large banks' activity. It is also how they are able to be market makers in sizeable amounts in FX - you can't do that kind of activity unless you have the information and business base to back it up - you just end up getting killed.
GJ
