I am not an expert while for me, ICO/IEO/STO the target are all the same, fundraising, but in different ways. STO looks more like stock, and similar to IPO, first, need to securitize the token/coin, get it legalized, doing STO in some STO platforms and the cost for STO is quite high as I know (at least 500K if I remember correctly? IEO is a no trend to partner with exchanges to get fund raised from exchanges users (Exchanges will also provide marketing/PR service to promote the projects. Some good IEO projects: Newton, Gifto, Bittorent, Historia network, Celer