MTL (mouth-to-lung) coils have higher resistance (usually 1.0ohm or higher) and are designed to replicate the sensation of smoking a traditional cigarette. They are ideal for vapers who prefer a tighter draw and use higher-PG e-liquids, including nic salts.
DTL (direct-to-lung) coils, typically with lower resistance (0.2-0.8ohm), are better for cloud production and sub-ohm vaping, offering a more airy draw and working well with high-VG e-liquids.
RDTL (restricted direct to lung) coils are a middle ground between MTL and DTL, and are often marketed as being optimised for nic salt vaping. This is because they feature a more open draw and increased vapour production, which makes the experience of using a refillable vape kit with a bar salt e-liquid that much more similar to using a disposable vape.