As you said, both are transceivers but the MMD has some extended feature compared to MM. The below are the differences between these two types of transceivers.
1000BASE-SX SFP transceiver module for MMF, 850-nm wavelength, dual LC/PC connector
1000BASE-SX SFP transceiver module for MMF, 850-nm wavelength, extended operating temperature range and DOM support, dual LC/PC connector
The MMD supports DOM whereas the MM doesn't suppor the same.
This URL will be useful for you. It contains the compatibility matrix and the IOS it supports.
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