On aircraft 45-002 thru 45-225 NOT modified by SB 45-55-6, the yaw damper must be engaged for all flight operations except take-off and landing. On aircraft 25-226 and subsequent and prior aircraft modified by SB 45-55-6, yaw damper use is not required.
No background on Type and no knowledge of the systems so my comment may be old cobblers ..
The AFM statement read as a certification thing appears not to have the implications suggested above. To me it reads quite simply -
(a) on premod early serials, you MUST have Y/D ON other than for T/O and landing. You can choose whether you have it ON for T/O and landing. More usually, one would just leave out the last bit.
(b) later and early postmod serials Y/D is optional.
In general, AFM limitations are to be read quite literally and ought never require any "reading between the lines"