A marketing asset is anything that can stand the test of time and improve your store's marketing.
The test of time means that they create on-going benefits for you after they are launched or that they can be re-used indefinitely.
Emails you sent to customers meet that, but an email that you send as a co-sponsorship with a partner doesn't.
The litmus test I use is: can this be reused again with very limited changes sometime in the next year?
The improvements to your store's marketing part is pretty self-explanatory but it should be clarified that there's no limit to the improvement. Both business-shifting big improvements and tiny incremental improvements both meet the definition. The level of improvement would change how valuable the asset is, but they are both considered marketing assets.
(And you might find that the tiny asset is actually more valuable, but that's getting ahead of things)
This definition of marketing assets is overly broad because there are so many types of marketing that can work for a store. Many of them are reusable or can be made reusable with some forethought.
The reuse is the hardest to get right though.
Part is building and keeping track of the assets you have which can be a mess. Part is knowing which parts to reuse, as many marketing campaigns fail or are mediocre at best and many marketers don't stick around and measure the final results.
That's why it's important to watch them and your customer behavior to see which ones have a positive improvement. Repeat Customer Insights includes a few ways to measure the different behaviors, along with a weekly email highlighting some of the big metrics to watch.
Use cohorts to find out who the best customers are in your Shopify store
Repeat Customer Insights will automatically group your customers into cohorts based on when they first purchased. This will let you see how the date customers bought would impact their behavior.