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It is not necessary that every letter in a part number has to actually translate to some real english word. These are usually just numbering chosen by the original manufacturer, and if the part becomes popular then others continue to use the basic part number.
no doubt T can mean Transmit, and R can mean Receive.
similarly in HT12D/E, the D means decoder and E means encoder, but the HT might be a totally meaningless sequence of letters.
Quite often though, the manufacturers use their company name abbreviation/ acronym as part of the number.
Hence HT12D might mean -- Holtek 2^12 series Decoder.