My question is, if my name is David Jackson Davidson and on some documents it is David J. Davidson , under which categrory will that fall?
Yeah, neither alias (which generally evokes negative connotations) nor nickname seem just right.
My guess is that the label applied does not matter so long as it fairly points to the way in which the name was used.
That is, my sense is that either alias or nickname is OK, that the drop down list is mostly to distinguish particular categories of name variation and name changes. Alias and nicknames being categories of informal name usage, in contrast to more formal name changes related to specific circumstances (marital name for example versus "maiden" name). I doubt there is any substantive import to the label, particularly in context (listing David J. Davidson as an alias should be obviously seen as a casual variation of the proper name and not evoke any negative insinuation that it was used as a false name or to conceal one's identity).
Otherwise, this is one of those items for which there may be a temptation to overthink things. But the way IRCC has composed this form, it is inviting if not demanding a lot of overthinking.
In particular, the restricted list of choices seems, to me, to be one more aspect in which the details of this form create a muddle. An "also known as" option, which embraces variations of name, would make more sense than "alias," particularly since "alias," in addition to the negative connotations, is often understood to mean a name which is different enough from one's proper name as to, in effect, be false, typically used to conceal one's proper name or even identity.
Prior forms allowed the PR applicant and citizenship applicant to describe or explain in his or her own words, and I listed a few which I described as "abbreviated forms of my proper name" or "variation of my proper name."
It is possible that IRCC does not really want these variations, but in terms of actual usage, particularly in the way such variations are typically used, including in particular names used with employers and vendors like names on telephone or utility accounts, such variations would seem more important than nicknames. Perhaps variations (such as with or without a middle name, or an initial in lieu of a middle name) could be considered the
same name by IRCC. But many people use an abbreviated form of their name for a telephone account, for example, and CIC/IRCC are known to use Canada411 to check the telephone numbers that applicants report in forms, so if they are looking for consistency, declaring the abbreviated name as one used will match the information derived from Canada411, and thus should assist IRCC's efforts to corroborate information provided by the applicant (which to my view is a big part of why IRCC wants all names used including nicknames).
For many years I was a licensed professional in a particular jurisdiction, and I do not recall why or how, but the name I was licensed under had no middle name or initial. Obviously, when I applied for my PR visa, that career history was significant, and it seemed obvious that I should give that variation of my name as one of the other names I used, at the least so the information I provided would be consistent with what CIC might otherwise discover in checking records. (It warrants noting that even a cursory internet search will show numerous similarly named individuals in the same profession even though I do not have a name that is all that common.)
An "aka" or "also known as" option would make sense. Some sources refer to these, aka and also known as, in stating the meaning of alias, or even "otherwise named" as a definition of alias. Alias in that sense would apply to variations of one's proper name including initials and dropped middle names. But many sources and dictionaries I am familiar with tend to emphasize the usage of "alias" in the sense of a false name, one typically used by criminals.
Among the many problems with the form, my guess is that item 6 is not a high priority, that substantial disclosure will readily suffice.