HYUNWOO K Here is the answer from Hollisgear I got last year. Hi, Sean The SMS100 was designed with primary application of sidemount. Mounting a tank to the rear of the harness will be secure, however when compared to a traditional BCD slightly loose. It is only recommended using the supplied cam bands, we do not recommend using a single tank adapter in this case as the support will be much less. If your goal is to use a tank and the tank feels wobbly on the rear, have you considered diving one tank on the side? This is very common as well. Thanks, Nick Hollis Hollis Gear

Monica s There is no difference they are the same items in the kit or individually.