Microsoft's SMI (Synchronized Accessible Media Interchange) format is somewhat old subtitle format that nowadays is found very rarely. Nevertheless, some subtitle editors still support and can convert it to something more common, such as the SRT or SUB format, which is what smi to srt conversion actually refers to.
Open SMI file Open SRT fileSmall tool for macOS, which allows users to convert ass, ssa, s2k, smi, sub files to SRT format