It is instant BUT that doesn't mean you can use it on-demand. PayPal will futher evaluate each potential purchas to determine if all qualifications are met for that purchase. If qualifies you will have PayPal Credit displayed as a payment otpion. If it is not there you just select one of the available payment methods.
PayPal does not disclose risk and other qualifications used.
If you want credit you can use on-demand, then just use your regular credit card.
