hmm assume reputation to be the same as prevalence here (even though it is not really the same English meaning...)
the reason for autosandbox to sandbox apps with 'The file prevalence/reputation is low' is based on data at avast servers that the whole avast community (everybody using avast) rarely use these apps (eg. emeditor).
basically, whenever u or other avast users open emeditor, avast notes the behaviour of emeditor and updates the cloud intelligence (filerep) on how emeditor behaves on systems. after a substantial period of analysis from sufficient avast users, avast will then deem if emeditor is safe/malicious and stream info from avast servers to avast clients (the antivirus on ur computer) to be released from being sandboxed/ added to virus database respectively.