What does 'Category 1 receiver' mean?

Category receiver numbers describes varying levels of receiver performance and is covered by EN300 220-V1. Category 1 is the highest and Circuit Design modules

STD-302Z 458 MHz
CDP-RX-07M/MP 434 MHz

satisfy the requirements for category 1.

An excerpt taken from EN300 220-V1 V3.1.1 (2017-02) is as follows:

Receiver category description

1
Category 1 is a high performance level of receiver. In particular to be used where the operation of a SRD may have inherent safety of human life implications.

1.5
Category 1.5 is an improved performance level of receiver category 2.

2
Category 2 is standard performance level of receiver.

For more details see the 'Receiver category 1' document.