A comment from Eyal @ bancor in Telegram

Eyal @ Bancor

There are two relevant configurations for tokens to utilize Bancor protocol:

1. Issue a smart token in the first place
2. Use a token-changer (a solution for any standard token)

The difference is that in (1) the liquidity is provided by a built-in, non-profit, automated market-maker, and the token issuer deposits the initial reserve (typically using the ICO deployed capital). In this set-up, the liquidity can be adjusted by the issuer over time (by gradually modifying the CRR), and there is no buy/sell spread.

While in (2), a token-changer smart token is deployed with two 50% reserves of BNT and the standard token. Anyone can deposit liquidity to these reserves by purchasing the smart token, which means that some conversion fee (fixed spread) might be applied - to attract additional deposits. Larger deposits would increase the liquidity (which is calculated as the price movement relative to the converted amount) by that - reducing the conversion cost.

In (2), unlike (1) the token liquidity level is determined by the free market, and cannot be adjusted by the issuer (which is the case in current exchanges anyway).