Advanced data layout, or ADL, is a function which looks at an input of raw data and returns one IPLD Node. This can come in the form of either a map or a string. The main purpose of ADLs is to provide the basis for many operations like path resolution with IPFS. Currently, you can't inject ADL code into an IPFS daemon over the REST API, but this may be possible in future releases with proper security measures. Another use case that comes to mind is creating shadow tree conventions where looking up one path would actually walk two trees and return corresponding data from each - however it might not be worth devoting complex code just for this purpose.