AACPlus is a combination of MPEG Advanced Audio Coding (AAC) and Spectral Band Replication (SBR) technology. The AAC codec is used for the coding of the low frequencies of an audio signal. SBR is used to recreate the higher frequencies out of the low frequencies together with some control information that is added by the encoder.
This combination can improve the coding efficiency of AAC by more than 30% at lower bit rates. AACPlus is able to deliver high-quality audio signals at bit rates of 24 kbit/s for mono and 48 kbit/s for stereo signals.
AACPlus is e.g. used by the DRM system for digital radio and XM Satellite Radio.
The combination of Low Complexity AAC with SBR is a special version of AAC+ which is called HE-AAC.