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From: https://forum.vcfed.org/index.php?threads/need-help-on-a-teac-fd-235hf-3-1-2-drive.1242497/#post-1307966
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A bit on Teac nomenclature. A=180K single-sided, B=360K double-sided, E=360K single-sided, F=720K double-sided, G=1.2M double-sided H=1.44M double-sided, J=2.88M double-sided.

So, an FD55F is a 5.25" 96 tpi drive that does double-density.
An FD55FG is a 96 tpi drive that does both double-density (720) and high--density 1.2M
An FD235F is a 3.5" double-sided 720K drive
An FD235HF is a 3.5" double-sided drive that can do both 720K and 1.44M
An FD235HG is a 3.5" drive that can do 1.23M (360RPM) and 1.44M (and also 720K; Teac stops the suffixes at 2 letters).
The exceptions are the FD235J and FD235S, which are multi-denstiy 2.88M and SCSI drives, respectively.

Additional letters, such as -R or -V relate to special features.