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Ilius B The combination of the sheet and Absorber Sponge Drop displays a high performance when playing a short stroke over the table, such as chop block. In addition, innovative attacking and defensive play are both possible, thus making best use of the characteristics of the long pimples, such as a low penetrating backhand with minimal spin.
PRODUCT HIGHLIGHTS
• Ilius B is the first long pimpled rubber with Absorber Sponge Drop, developed by Butterfly
• This new hard sponge really shines close to the table for blocking, chop blocking, and drop shots
• To accentuate the long-pip effect, the pimple shape is as thin and high as ITTF rules allow
• Designed for shot/spin variation and easy attacking, making it easy to keep the ball low
• Recommended for players that want both innovative attacking and defensive play

SPECIFICATIONS
• Surface: Long Pimpled (Long Pips)
• Stability: 6
• Variation: 7
• Hardness 48

Ilius S. The combination of the sheet and Absorber Sponge Heavy makes it easier to play a push stroke and to return an opponent’s attack with a low defensive stroke with heavy spin or with minimal spin. The rubber is recommended for defensive players, enabling subtle changes as well as stability in rallies.
PRODUCT HIGHLIGHTS
• Ilius S is the first long pimpled rubber with Absorber Sponge Heavy, developed by Butterfly
• This new hard sponge allows for exceptional control on chops and pushes, even when receiving powerful shots
• To accentuate the long-pip effect, the pimple shape is as thin and high as ITTF rules allow
• Designed for spin variation and ease to block an opponent’s return, all with a low trajectory
• Ilius S is recommended for defensive players, enabling subtle changes as well as stability in rallies

SPECIFICATIONS
• Surface: Long Pimpled (Long Pips)
• Stability: 7
• Variation: 6
• Hardness 45

The way I would remember is S is for Slice for away from the table choppers & B is for Block for close to the table blockers

But I also have to wonder if the sponge name for Absorber Sponge Heavy and Absober Sponge Drop got swapped because Hevey has a lower hardness of 45 than drop with 48.
Then again may be it is the way they just named them & has no correlation with hardness , and so I may be overthinking it.

Here is the detailed comparison at official Butterfly webpage