All devices share a default global Trust Center Link Key. This is an APS layer key and is the first key to be used
when joining a network, if no other link key is specified.
这是TI手册里面的描述,意思是如果没有设置别的link key(install code),那么Global TC Link key就是第一个key用于加入网络
When a device is factory new and receives the APS Transport Key command, it loads the key to be attempted for a centralized network (Install Code if loaded through BDB API or Global Default Centralized Key if no Install Code is set).
如果不设置install code key,当然就默认使用Global key了,实际上就是个兼容设计。