This is the channel BaseLine Threshold register which is register  0x1n64

When the Peak Averaging Window in the DPP_CTRL register is "0" it defines the absolute baseline value.

Otherwise, according to the numeric value set into the DPP_CTRL register it defines the acceptance band:
when the input signal does outside the +- blthr interval the corresponding samples are not included in the
input signal baseline mean calculation. In other words, when abs(baseline-signal) < blthr the baseline mean
is calculated