This is a question that you will need to ask your child's school administrator, because the answer may differ from school district to school district. An example of a typical excused absence is for an illness where a doctor's visit was required, or for the death of an immediate family member. There should be a list of acceptable excuses. An unexcused absence would be any circumstance not on the list. One example might be to attend a family wedding or take a family trip.
According to the SF Examiner Article:
"An unexcused absence is when a child is not in school for reasons other than those approved, which include illness, court appearances and funeral services for a family member."
Thus, even if you provide a parental note, but take the child out for reasons not listed above, it counts as an unexcused absence for the district. In this case, unexcused absences affects school funding because the district is compensated by the state on a per student per day basis.
If your child was out for 5 days on a vacation and the district receives $25/day/student then the district would not receive that $125 in funding. This affects school funding as well because schools receive funds from their district.