Describe or document the following kind of decisions:

• critical or important for the system
• influencing important quality attributes
• unconventional (“off the beaten track”)
• risky
• with expensive consequences
• with long-lasting effects
• affecting either
  • a large number of stakeholders
  • very special or important stakeholders
• that took a long time or much effort to decide
• astonishing

(document) “architecturally significant” decisions: those that affect the structure, non-functional characteristics, dependencies, interfaces, or construction techniques.

Quoted from Michael Nygard