Good comments above. I would just add IOI's are typically non-binding whereas LOI's get closer to being binding (or at least have a number of binding provisions such as confidentiality, exclusivity, break-up fees in the rare case, etc.). Each process is different depending on the process letter that advisers set, but typically IOI's will include a wider range of valuation and high level terms whereby by the time LOI's come around, the valuation will be set (or at least within a precise range) and terms are often defined in much more detail.

I find that IOIs are just to see how much interest there is in the market. Sometimes the broker is unfamiliar with the industry so they drag out the process a bit more and ask for the IOI first, but IOI are good to get a lot of questions out to the seller without a binding agreement. LOI's are typically when the seller is trying to narrow down to one buyer and they are ready to sell. Expect the process to be at least 90 days from the IOI or longer depending on the due diligence required.