The deadline indicates the date by which suppliers must submit their response to your project. After the deadline expires respondents may no longer edit their answers or submit their response. You do not need to specify a deadline, and you can always modify the date later.
A deadline is defined for each respondent organization. There is also a deadline field on the parent Project. When you change the Project's deadline, the system will offer to synchronize the deadline for each respondent.
The Project deadline constrains the actions of the Project issuer (the buyer). For example, it's not possible to invite new respondents after the Project deadline has passed.
The Response deadline constrains the actions of the respondent - e.g. not being able to submit after the deadline has passed.