Hi,
I don't know why some objects do not use ACTVT. One of the reasons could be that developers were not happy with activities in TACT and created separate field.
Regarding ACO_ACT_S. It uses domain ACO_ACTIVITY_SUPER. If you open this domain in SE11 then you will see that it allows only 3 values (ADMIN, WRITE, READ). Why developers did not use activities 33,34 and 70 of field ACTVT? No idea.
Generally, you could have a field with domain that does not restrict values and no table assigned to this field. In this case the only 2 ways how to figure out values for this field would be to tracing application or reading of code.
Cheers