Analyse following RPM build log snippet. Describe contents accurately, without speculation or suggestions for resolution.            
Your analysis must be as concise as possible, while keeping relevant information intact.
Snippet: 
app_processes.proto: This file contains proto3 optional fields, but --experimental_allow_proto3_optional was not set.
File `app_process.proto` contains proto3 optional fields, but flag allowing these fields `--experimental_allow_proto3_optional` was not set.