A soft federation is when maintained schools formally collaborate
This can be 2 or more schools. They can arrange for any of their functions to be carried out jointly, such as through joint committees and joint board meetings.
But individual boards still have legal responsibility for all decisions made on their behalf and they don't form 1 governing board. You must make sure your boards receive regular reports and signed minutes from any joint committees you establish.
This is different from a hard federation, where 2 or more maintained schools share a single governing board.
This is outlined in regulation 3 of The School Governance (Collaboration) (England) Regulations 2003, schedule 2 of The School Governance (Roles, Procedures and Allowances) (England) Regulations 2013, and in section 5.8.1 of the maintained schools governance guide.
It can be a time-bound arrangement