I don’t know about the semantics of the description, but in 551M:

LDRA throws P1556 (underboost)
LDO throws P1555 (overboost)

LDRA will also throw P1556 if the ratio ps_w/pvdkds_w exceeds the threshold from KFVPDKLD.

i see you all guys are still confused about my situation here
here is the story my car is 2002 s4 2.7t tiptronic swapped to manual transmission a year ago, the original ecu  was aa-box flashed to m-box, i did some modes on that car like rear os delete egt delete and rebuild  both turbos.
since then car runs just fine no issues at all no chek lights , but about a week  i start smelling gas i poped the hood i found all injectrs leaking gas evry where i tought its maybe the injectors seals needs to be replaced and i did replaced all injectors seals and when i did that i think i installed back the check valve for the evap valve  backwards because since then the 5psi issue start to happen with no fault codes because i never get my s4 under load or hard driving so maybe  that's why she is not throwing fault codes
i took my friend advice pushed my s4 hard on highway till the check light turned on returned back to my place plugged in vagcome and found the P1556  Charge pressure control; negative deviation
i fx it now every think good but my injectors still leaking
replaced the fuel rail same thing all injectors leaking so i thoaght its mybe the injectors are bad.
 i got 06a9060311.8t injectors in my garage so i told to my self lets try them so i puted  them on ,car start fine no leaking gas any more so i orderd ev14 550c injectors and that's it end oof the story

no i didn't check the wastegate with gauge but i did test them with compressed air they are working properly and about  the n75 valve i clean it with wd40, installed back checked all pressur hoses from the n75 to wastegate all good i will do some logs but what's about the leaky injectors are thy failing like that leaking like the are cracked