The inner diameter. Think of the Inshore as the “medium” and the Big Bass as the “large”. Inshore Cushits will fit rod butts 1 1/8″ in diameter or smaller, and the Big Bass Cushits will fit rods greater than 1 1/8″. So,

• 1.125″ or less rod butt diameter = Inshore
• Greater than 1.125″ = Big Bass

Here’s a video to explain it even more!