There can be different soil material that can be used as a backfill for as long as it complies with the structural requirement. But the best material used for backfilling a retaining wall are well-graded gravel.
Gravel are best material because of its geotechnical properties. It has small void ratio, because of this, it retains less water compared to other soil material. When a soil material absorbs a good amount of water, it can contribute to the additional pressure the retaining wall resist which exceeds its designed strength.
When this happen, structural damages may happen and contribute to damages. Gravel on the other hand, retains less water which means lesser lateral pressure. Others, may also consider other coarse-graded soil. These are gravelly and sandy type of soil and ranges from clayey sands to high quality gravel-sand mixture.
Coarse-grade soil are the most popular because it has high quality properties and exhibits slight to no plasticity. These type of soil are easy to compact and provides a good support for the foundation.