It is the billing layer for a lot of services
If somebody subscribed to a music, video or productivity service through an Android phone, the Google account bills them. A code from that service often cannot apply; Google Play credit can.
This is why several other pages on this site point here. When in doubt about who bills the recipient, this is frequently the answer.
What the balance covers
- Apps, games and in-app purchases.
- Films and books where sold in that market.
- Subscriptions billed through the account, including renewals.
- Storage plans billed through the account.
It does not cover hardware, and it does not reach anything billed outside the account.
Region
Google accounts are tied to a country, set by the payment address. Credit is country-specific and will not redeem onto an account registered elsewhere.
Changing an account's country is restricted and generally cannot be done with a balance sitting on it — the same dead end as the gaming platforms. Get the country right the first time.
Family libraries
Where a family group shares purchases, credit still belongs to the individual account that redeemed it. It does not pool. A code given to one family member cannot pay another's subscription.
Amount
Whole billing periods if it is aimed at a subscription; anything at all if it is general spending money, since the catalogue is wide enough that nothing is stranded.
Asking for one
Country and amount. See gift cards for subscriptions and the software guide.