Short answer
Rope dyeing gathers warp yarns into thick ropes and dips them through multiple indigo vats (producing deep ring-dyed yarns that fade beautifully), whereas slasher dyeing passes sheet warps through indigo vats in a flat ribbon layout, which is faster but offers slightly less deep core-white contrast.
Denim mills in Ahmedabad utilize both dyeing methods depending on the target fabric grade:
Tell us what you're looking for and we'll point you to the right market or supplier.