IEO stands for INITIAL EXCHANGE OFFERING,  as the name goes it takes place in exchanges. Also, just like ICO and STO, IEO is just another form or medium of raising funds for upcoming projects, but the only difference is it takes place in exchanges and as such is free from scams and fraudulent activities found in ICOs. Also, in the case of IEOs, in order to participate, the user, participant or investor needs to hold the exchanges native token. Furthermore, the major advantage IEO has is that listing and trading is certian which takes place within few days unlike in ICOs where the team takes time to list their token or even ends up not listing.