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The length bs used in the bending term is the the length of the shear resisting elements, between the outer edges of the end stud assemblies.
The length beff used in the hold-down displacement term is the length between hold-down force and compression force locations i.e. the same distance that is used as the moment arm of the reaction to the overturning force. Refer to that topic for the calculation of this length.
Note that this is contrary the definition of b in SDPWS Equation 4.3-1, which refers to the "shear wall length', but using the overturning moment arm is a more accurate way of converting vertical hold-down displacement to rotational shear wall deflection due to overturning. SDPWS Figure C4.4.3.4B in example C4.3.4-2 shows beff as the appropriate value to use, and this was confirmed in discussions with AWC.
Perforated Shear Walls
For perforated shear walls, as per SDPWS 4.3.2.1, the lengths bs used for bending and beff for hold-down displacement are replaced by the sum of the lengths of the full-height segments Sbi, and one deflection is calculated for the entire wall. Note that the summation of full-height segments is also adjusted for narrow segments as per 4.3.3.4.
FTAO Walls
For FTAO walls, the lengths b and beff used in the deflection equation are based on the full-height segments between wall openings, however, one deflection is calculated for the entire wall by averaging the segment deflections as described in Deflection of FTAO Walls.