We all know that Google AdSense works by serving targeted Ads to the websites that are enrolled with the Google AdSense program. However, there are instances when no targeted Ads are available for a website e.g. the website might be catering to such a topic or subject for which no relevant Ads/advertisers are available. In such a case, Google serves alternate Ads i.e. content of your choice.

If you have not specified any content as Alternate Ads, Google AdSense program treats PSAs i.e. public service Ads as alternate Ads and serves them on your website. These PSAs are charitable Ads that don’t contribute to your revenue. So, unless you are looking to do charity (which could mean lessening of revenue for you), you must specify Alternate Ads upfront.