1 boundary position sounds like marketingspeak to me. Presumably they are attempting to use boundary reinforcement to exaggerate the subs low frequency extension. This is BS because, a) nobody knows how they came up with the spec and b) pure boundary reinforcement does not strengthen lower frequencies more than it does higher frequencies (within the sub's range). I'd GUESS that the 28 Hz number is intended to be a -10 dB point but there's no way to know for sure unless the mfr says what it means. In a small enough room, cabin gain will compensate somewhat for the rolloff of the sub's output, but cabin gain increases at (theoretically at most) 6 dB per octave while output is falling at 24 dB per octave, so it can never catch up . . .
2 the fb SHOULD be port tuning (ie the place where the enclosure's port output is strongest. IIRC, you can expect a ported sub to roll off at 24 dB per octave below port tuning, so by about 22.5 Hz, its output (in the best case) is 14 dB lower than at 45 Hz, or a little more than 'one notch" less than half as loud (sort of - - our hearing is less sensitive at low frequencies and while -10 dB is generally considered half as loud in the midrange it might really be, subjectively, more like a fourth as loud at 45 dB . . . . IOW no significant output very far below the tuning frequency.
I'd guess the -3dB point to be in the range of 40 Hz. But "-3 dB compared to what?" is a legitimate question that you simply cannot answer with the information given.
"Boundary Position" looks like the company's terminology for Frequency Response. Looking at the specs that seems logical.
I would guess that the LF Response spec is the frequency with maximum output.
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