what is ‘Is_trustworthy_on’ option in SQL server

This setting is_trustworthy_on is used to control the access of CLR based stored procedures, UDF’s(Scalar & Table Valued), Triggers, Aggregates, User defined data types accessing external resources when these are executed under
SQL server service accounts

What are external resources?

External resources include lot of things starting from accessing a network shared locations, all the

Supported Namepaces by SQL server are not considered external resources. When coming to resources,

some of resources where SQL server considers them as External resources include

writeto event logs, use network services like DNS and Active Directory, create outbound
network connections including Web access (port 80 and 443) and SMTP mail access (port 25), and access HKEY_CLASSES_ROOT, HKEY_LOCAL_MACHINE,HKEY_CURRENT_USER, HKEY_CURRENT_CONFIG, and HKEY_USERS registry keys

when IS_TRUSTWORTHY_IS is set OFF and if CLR is trying to access these resources even under UNSAFE mode then then execution
results a falure