What is the meaning or definition of scalable audio coding?

tools like BSAC and SBR are ones for these purpose?

Is there any other tool?

AFAIK, a "scalable" (audio/video) stream provides a base layer (lower resolution, lower quality) and one or more enhancement layers or at least a specific encoding that supports partial decoding to allow bandwidth and/or complexity savings.

For audio scalable can mean the ability to decode at different sampling rates or at a different channel configurations. (mono-to-stereo enhancement layer for example)

For video it would be:
Scalable temporal resolution (frame rate)
multiple image resolutions
multiple quantization levels (SNR scalability)

MPEG 4 Audio provides different shemes concering mono/stereo/multichannel scalability and sampling rate scalability. SBR is a tool for sampling rate scalability.

HTH,
Sebi

As far as I know:
SBR wasn't part of MPEG4 AAC scalable profile as default, but by it's nature itself (it is backward compartible) it could be considered as freq. scalability tool.