It means the sensor can never read higher, nor can the ecu request higher.
it is covered in depth in the various 5120 threads and mentioned several times in the s4wiki.
with stock DSLOFS values, it is theoretically possible for the ecu to request slightly more boost than the sensor can read, thus causing the ECU to be confused and continuously add duty until it maxes out.
https://s4wiki.com/wiki/Tuning#Specifying_requested_boosthttps://s4wiki.com/wiki/Tuning#PID_vs_ME7_boost_limitationshttp://nefariousmotorsports.com/forum/index.php?topic=3027.msg29814#msg29814