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

Quoted from Michael Nygard

Don’t document every tiny development decision - but concentrate on the following:

• 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