I have 40 years of evidence that tells me you are making a huge mistake trying to do this on your own. I have some "other advice" I think would be valuable, but you likely will not appreciate it. GET AN ATTORNEY.
It seems you have been rejected for descriptiveness if you are being asked to provide proof of secondary meaning ("secondary source" in your messed up lingo). And you think that needs to be on goods other than those you are trying to register? Not hardly. You say you "understand it to an extent"--yes, that would be to a miniscule extent, i.e. NOT AT ALL.
When in a trademark legal conflict with a trademark examining attorney, you are totally outclassed and outgunned unless you have a trademark attorney working for you. Most of the attorneys that post here can totally outgun and outargue the average trademark examining attorney, but you - well you are at the point where if you don't get an attorney you appear headed for loss of your brand name. Since that is likely a major key to your business, don't you think it might make sense to get an attorney so someone that has a clue what secondary meaning means is responding to the rejection? You are NOT going to get enough hear to avoid hiring an attorney, but you might here find one that can pull your rear out of the frying pan it is in. Get mad at me if you want, but you need someone to knock some sense into you so you get an attorney before you totally screw this up (if you haven't already.)
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
First, why was the logo rejected - generic? It is difficult to provide specific advice without reading the full Office Action.
Regarding the request to submit evidence, have you used the logo on other products, for example, clothing?
If you have used the logo on other products, do you have advertisements, purchase orders, etc. that show that the logo is associated with your business?
Attorney Perez properly referred you to the instructions on how to establish that your logo does, in fact, indicate the source of goods or services that you offer to the marketplace rather than simply being a nifty design that you happen to put on stickers and clothes. [Attorney Burdick was wrong to assert that your use of the phrase "secondary source" was an erroneous reference to "secondary meaning." The two concepts are different.]
The problem for you is that it does not appear you use the logo as a source identifier for any products other than stickers and clothes. Those intructions, therefore, won't be of any help to you at all.
IF you submitted a sticker as your specimen of use you could try to submit a new specimen showing the logo as you use it on clothes -- USED AS A MARK, not just displayed as a nifty logo in the center of the shirt.
Those two options are the only ones that I can think of at the moment. If this logo is important to your business -- which it should be if it's the main way that you brand your clothes -- the you have no choice but to hire a trademark attorney. Good luck.
The above response is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.
One example of secondary source is a university that sells t-shirts, mugs, etc. emblazoned with the university logo. The logo on a shirt would be considered ornamental because the university is not considered a source/producer of t-shirts. The university is, however a source of educational services. By showing use of its logo in offering educational services, the university can show a secondary source, allowing the logo to be used as a mark for t-shirts and other goods.