There are four mutual fund share classes, A, B, C and No Load. There are other share classes but most of them are probably not testable with the exception of an institutional share class, which have lower fees but very high minimums in the millions.
Investors work with financial advisors when they buy A, B or C chares. Most people buy no load funds directly from fund companies like Vanguard and Fidelity.
Every load fund has both a sales charge and a 12b-1 fee. Sales charges are assessed when an investor buys or sell shares and are capped at 8.5%. 12b-1 fees are are used for distribution expenses like advertising and marketing and are capped at 0.75% of net assets. There is also another part of the 12b-1 fee that is for account servicing, which is capped at 0.25%. Every fund: load and no load, has a management fee, which goes to pay the investment adviser. The management fee and the 12b-1 fee are a large part of the annual expense ratio.
A shares are the original share class. They have an upfront sales charge and a low 12b-1 fee, which is usually 0.25%. They are most suitable for people with large amount money to investment and/or a long time horizon. If you have a a lot of money to invest, you will pay a much lower sales charge due to break points. The long time horizon is partly related to how low 12b-1 fees will save you money compared to B and C shares over time.
B shares have a back end sales charge, which we formally call a CDSC or a contingent deferred sales charge. The charge is contingent upon when you sell the fund and if you hold it long enough, there will be no sales charge because it turns into an A share. What made B shares expensive was the 12b-1 fees, which are typically at the maximum of 1%. I said this in past tense because B shares, for the most part, have become like dinosaurs in that they have become extinct but may still be tested.
Regarding suitability of them, you would literally have to work out the math of all the charge to see if it was more or less expensive compared to other share classes. I would say you would never buy a B shares if you have a short time horizon because the back end sales charge would be very high.
C shares or what some people call level load, have a short CDSC of about 1 year and the charge is usually 1%. However, their 12b-1 is at the maximum. The reason they are sometimes called level load is mostly due to the 12b-1 fee. Until the past few years, C shares never converted into A shares, so investors paid the 12b-1 fee for as long as they held the fund. This has changed recently after complaints and pressure from the SEC and FINRA. Now C shares eventually convert into A shares but it could take up to 10 years.... C shares are most appropriate for people with a small amount of money to invest and a short time horizon.
No Load Funds have no sales charges but can have a 12b-1 fee of up to 0.25%. From a suitability perspective, if a customer has the time to do the research and/or wants fees to be lower, a no load would make sense