What is ASIN?
ASIN stands for Amazon Standard Identification Number.
It’s a unique set of 10 alphanumeric characters.
This item identifier is assigned by region specific Amazon and it's regional partners. It is used for product identification within the Amazon category specific product catalog.
While searching for items, that you'd like to sell in US, you want to make sure that you use the ASIN(s) from Amazon US and not the other domains of Amazon such as Amazon MX, UK, CA, EU etc.
ASIN(s) are specific to their region. Therefore you cannot use an ASIN from a different region to list on Amazon US.
[Whether for your existing listing or to grab details into SellerChamp]
How to look up product ASIN(s)
ASIN(s) can be located from different places:
The Amazon product web address
Within the product details
ASIN lookup software and tools