This all depends on what asset the IOU represents and by extension what it can be used for.
If in the case of USD or EUR etc they are accepted for payment of goods or services by a third party who is not the issuer then they are e-money.
In this case the issuer has to be either authorized or registered as an Electronic Money Institution (EMI).
The key difference between this and a deposit account is that no benefit can be given for holding on to the e-money over time such as interest.
They are redeemable for face value minus any reasonable charges.
Ripple IOUs can also be for bitcoins for example. However since they are not considered legal tender - you cannot pay your taxes in bitcoins for example, they are considered barter or the same as gold. These IOUs are not e-money.
If I were to issue Ripple beer IOUs which could be exchanged in any pub worldwide for a beer these would still not be e-money if they were only exchanged for beer since this is a closed market. I could not use my $50 worth of beer IOUs to buy a book for example. If book stores and other establishments started accepting them at face value then they would become e-money USD.
The UK FCS does a good job of explaining