Which is the prefix

Hi,
I just have installed the plugin.

which value should I introduce in Google Merchant Center prefix ?

I´ve created the feed Importing by Google Docs (xls).

I´ve not used this value. Which is the default value? I´ve used “g” but I dont know if is ok.

url is http://infanity.es

Thank you.

https://wordpress.org/plugins/woocommerce-google-dynamic-retargeting-tag/