a pattern like 000 -> 100 -> 110 -> 111 -> 011 -> ...

and goes on... instead of just checking all 0s or all 1s the patterns check also for transition (the memory cell may fail 0->1 but not 1->0)

there are other patterns... diagnostic programs for PCs usually have many options for choice, depending the time available for testing, sometimes testing all patterns (and sometime the faillure happens just in one).

hth

orlando