Congratulations! You have reached one of the leading sites for information about determining requirements for systems, whether for changing something existing already (brownfield requirements), creating something new (greenfield requirements), or checking an existing system for completeness, consistency, or correctness.
'Systems' mean any kind of human made system. The thrust of much writing and activity about requirements is really about creating software; software in this sense refers to the instructions needed by digital computers to support wider system needs.
However, we are using the term 'requirements' in a very broad sense, and indeed some human activity systems may not use any software or computer support.