Writing
Writing is the highest bar at Proficient and Superior: 10 and 12, against 8 to 10 in the other skills. Superior Writing, 12, is the top of the CELPIP scale. If you are aiming for Proficient or Superior, plan your time around Writing.
Functional
Functional English is the level secondary applicants are often asked for, and it is the only one read from the overall score rather than skill by skill. When a test is used as evidence, Home Affairs generally requires it from the 12 months before the visa application.
CEFR
For comparison, on celpip.ca's chart CELPIP 7 to 9 sits at CEFR B2 and 10 to 11 at C1. For how CELPIP levels line up with CLB, IELTS and PTE Core, see the CELPIP score chart.