Flexible pledge means the goal of the fundraising campaign is not a mandatory requirement of being successful. That is, promoters can receive all the funds they raise during its campaign on String Funding.
All-or-Nothing is a rule that promoter must reach a funding goal in order to receive the money that they raise. If All-or-Nothing campaign has not reached the funding goal, the fund raised so far will be refunded to backer's String Funding account.
To avoid unnecessary charges by financial institutions, refunded funds will stay in backer's account.