What is the difference between an excused occasional omission and habitual disregard of sunnah?

Prescribed Acts and Prohibitions: Fard, Wajib, Sunnah, Nafl, Haram, Makruh, Azimah and Rukhsah

• The text makes allowance for excuse or occasional omission but may blame habitual neglect
• It equates every temporary omission with denial of fard
• It treats all omissions of sunnah as identical to omitting nafl
• It places no emphasis on maintaining Sunnah