O que significa o termo sobrecarga de NAT; isso é PAT?

Sim. NAT overloading is PAT, which involves using a pool with a range of one or more addresses or using an interface IP address in combination with the port. When you overload, you create a fully extended translation. This is a translation table entry containing IP address and source/destination port information, which is commonly called PAT or overloading.

PAT (or overloading) is a feature of Cisco IOS NAT that is used to translate internal(inside local) private addresses to one or more outside(inside global, usually registered) IP addresses. Unique source port numbers on each translation are used to distinguish between the conversations.