Here's the official documentation on the formula - https://help.anaplan.com/138ee6b0-34a3-4654-b944-f655e6b28ad8-ISACTUALVERSION
As per the article, ISACTUALVERSION returns TRUE for Actual Version and FALSE for all other Versions.
However, I observed that the actual output is different when you use Time as a Dimension.
With Time as one of the Dimensions, the output is a function of Switchover month for each version – TRUE for periods before the switchover period and FALSE for rest, for EVERY Version, not just Actual Version.