You can add an N/A (not applicable) column to any question type that uses weights:
When respondents select the N/A answer option, that answer will not be factored into the question's weighted average calculations in the Analyze Results section.
To add an N/A column to a Matrix/Rating Scale or Ranking question:
When you add an N/A column to a Matrix/Rating Scale question (with the Use Weights option applied), an N/A column without a weight associated with it will be added to the column choices.
If a rating item does not apply to a respondent, an N/A column will give the respondent an opportunity to essentially opt out of the question, instead of choosing an inaccurate answer.
When you add an N/A option to a Ranking question, it will add an N/A checkbox at the end of each row. If a respondent selects an N/A checkbox, that row will be greyed out and will not be included in that respondent's ranking order.
Respondents answer a Ranking question by dragging and dropping their answers in order of preference, so you want to make sure that even the lowest ranked options still apply to that respondent. If a respondent doesn't have the option to declare that a ranking item does not apply to them, they will be forced to include that choice in their rankings anyway, which might produce bad data.
You can also add an N/A option as an additional answer choice to other question types, like Multiple Choice, but the system will not automatically remove those choices from analysis.