Digital Signature

can anybody tell me what is the use of Digital Signature ?

in qm view i assigned 000022 Digital signature at usage decision

to QM material auth in General data

while making usage decesion it Requires digital signature user id & password .but allow usage decesion from same user id & password of sap log in .

if i want to change the password without changing my sap log in, password &only one user id .How can I config it .

Thanks in Advance

Regards

Sanjay