Typically it occurs when a deal is in progress or is being contemplated and you want to do a deal a certain way and find that there's a technical roadblock to doing it that way, one that in policy terms really shouldn't be a problem but is. And because the dollars are so big and you're doing the deal or would like to do the deal.... I guess you could go around this roadblock, but it would be extremely costly.
The first step in a comfort letter is typically that you will contact the CRA to find out whether they are aware of this issue and have an administrative policy with respect to it. If they say no, the law is what the law is, then the next approach is to write a letter to Finance indicating that there is this technical anomaly and that there appears to be a completely unintended result, in tax policy terms, that frustrates an otherwise normal commercial transaction—