High Efficiency Video Coding tiers and levels

High Efficiency Video Coding tiers and levels are constraints that define a High Efficiency Video Coding (HEVC) bitstream in terms of maximum bit rate, maximum luma sample rate, maximum luma picture size, minimum compression ratio, maximum number of slices allowed, and maximum number of tiles allowed. Lower tiers are more constrained than higher tiers and lower levels are more constrained than higher levels.
Tiers.
The HEVC standard defines two tiers: Main and High. The Main tier is a lower tier than the High tier. The tiers were made to deal with applications that differ in terms of their maximum bit rate. The Main tier was designed for most applications while the High tier was designed for very demanding applications.
Levels.
The HEVC standard defines thirteen levels. A level is a set of constraints for a bitstream. For levels below level 4 only the Main tier is allowed. A decoder that conforms to a given tier/level is required to be capable of decoding all bitstreams that are encoded for that tier/level and for all lower tiers/levels.