A. Wymore postulates¹⁾ there are 6 categories of requirements:

• IOR is the I/O requirement,
• TYR is the technology requirement,
• PR is the performance requirement,
• CR is the cost requirement,
• TR is the trade-off requirement and
• STR is the system test requirement.

Each system must fulfill these requirements.

Taken from NASA’s ISS Crew Transportation and Services Requirement Document²⁾

In the context of a contract:

• “Shall” is used for binding requirements that must be verified and have an accompanying method of verification;
• “Will” is used as a statement of fact, declaration of purpose, or expected occurrence;
• “Should” is used to denote an attribute or best practice which must be addressed by the system design.

When used within the context of a reference document under an agreement, the verbs shall, will, and should are only intended as informational and are not binding.

In some cases, the values of quantities included in this document have not been confirmed and are designated as:

• “To Be Confirmed” (TBC) – still under evaluation,
• “To Be Determined” (TBD) or “To Be Supplied” (TBS) – known, but not yet available,
• “To Be Resolved” (TBR) is used when there is a disagreement on the requirement between technical teams.

When a change in a noted characteristic is deemed appropriate, notification of the change shall be sent to the appropriate review and change control authority.