LD3 is the Output Unit number of the Responder output section. It specifies which Output Unit of the device should react as a Responder to the link record. For the vast majority of Insteon devices the first or only Output Unit number has always been 00 (for years). The KeypadLinc is one exception in that button Output Unit numbers run from 1-8 (rather than 0-7). This is the only single output Insteon device I am aware of that is using an Output Unit number of 01.
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