Object distance

One symbol, two measuring points, and the AP CED is explicit about both.

For a mirror, EK 13.2.A.7 says the image location depends on the focal length and the distance between the object and the surface of the mirror. For a thin lens, EK 13.4.A.5 says it depends on the focal length and the distance between the object and the midline of the lens. Same $s_o$, different reference surface, and the lens one only makes sense because the lens is being treated as thin.

Where it appears. The AP Physics 2 sheet prints

and $\lvert M \rvert = \lvert h_i/h_o \rvert = \lvert s_i/s_o \rvert$. The sheet's symbol list defines $s$ as position and $f$ as frequency or focal length. Note that the CED's own instructional material writes the same quantity as $d_o$ in one sample question, so both notations turn up and neither is wrong.

Signs. EK 13.2.A.7.i and EK 13.4.A.5.i say only that the focal point, object and image locations follow sign conventions applied relative to the optic itself. The CED does not print the sign table, so state the convention you are using before you use it. Under the usual one, $s_o$ is positive for an object on the side the light comes from, and the sign that actually decides real against virtual is the sign of $s_i$.

Rearranged as $1/s_i = 1/f - 1/s_o$, a plot of $1/s_i$ against $1/s_o$ is a straight line of slope $-1$ whose vertical intercept is $1/f$. That is the standard lab route to a focal length.