The DfE's guidance on recording absence due to strike action in schools recommends using the Y code to record absence due to strike day closures.
The Y code is not counted as a possible session.
You can see this by comparing the total sessions on a student's attendance tab:
|Was required to attend on a strike day:||Was NOT required to attend on strike day:|
The impact of this can also be seen when comparing attendance in classes for the strike day.
If only two children were required to attend on that day, and one of those was unwell and an I code was used for the two sessions as shown above, attendance for that class for that day would be 50% (2 present marks out of a possible 4 marks, multiplied by 100 to convert to a percentage) as shown in the Attendance by Year/Form report below.