25.40. DO UNTIL condition
This command implements a looping construct that loops the code between the line DO UNTIL and LOOP util the expression following the keywords on the loop starting line becomes true.
DO UNTIL expression ... commands to repeat ... LOOP
The expression is evaluated when the looping starts and each time the loop is restarted. It means that the code between the DO UNTIL and LOOP lines may be skipped totally if the expression evaluates to some TRUE value during the first evaluation before the execution starts the loop.
This command is practically equivalent to the construct
WHILE NOT expression ... commands to repeat ... WEND
You can and you also should use the construct that creates more readable code.