This value indicates whether this particular row in the svcAtmFailedCall table currently holds valid, meaningful failed-call data, per the description, which explains that all entries start out invalid(2) and, because the table operates as a FIFO of fixed size N, every entry becomes and remains valid(1) once N failures have accumulated on the switch. This means an admin should check this field before trusting a given row, especially shortly after a switch reboot or table reset, since invalid(2) rows simply haven't been populated with a real failure yet rather than representing an actual empty-but-successful call. Once the switch has logged N or more failures overall, every row will read valid(1) and this field stops being informative on its own.