I'm not sure how to ask this, but if you want to advertise on ClixSense (through the ClixGrid or the ViewAds sections),
would it be allowed to advertise a non-PTC site? Since that's all I ever see.

For example: If I was selling T-shirts online, would I be allowed to use ClixSense advertising or is it strictly limited to only advertising other PTC websites?

You can advertise Non PTC sites at ClixSense. However, if the site content includes inappropriate content such as drugs, adult, weapons etc, your ad will not be approved.

PTC sites and related sites do best. However you can get great results promoting non-related affiliate products on clixgrid ads. Best ones offer lifetime code and higher percentage of sales. Also helps if so addictive must buy it very often. If in doubt just do test run. I did for week after saw results started buying the 6 month ads. If profitable on week long run than be even more so on the 6 month long ones as they cheaper per day longer buy. Also cheaper if buy in bulk.

Longer affiliate offers tracking means more profitable. Some do it for less than day, others week, months, year and best its lifetime meaning first affiliate showed person link gets the sale for life. Next percentage get on first sale and recurring sales. Some offer single digit cut others offer in 90 percentage. Higher is better but low enough they still turn profit. No point otherwise do not want them going out of business or being half year or longer late paying. Ideal product they spend few hundred or more monthly and so addictive they keep buying same amount weekly or monthly. Know I spend few hundred weekly on my sweet tooth buying chocolate. I buy the expensive much better European chocolate not cheap mass produced American garbage variety.

It looks like porn, dating and HYIP might be the only 2 niches that might survive online after all. It's time for Bernie to make the deliveries - maybe in 4 years............