Reference:

Explanation
The term 'rootkit' originally comes from the Unix world, where the word 'root' is used to describe a user with the highest possible level of access privileges, similar to an 'Administrator' in Windows. The word 'kit' refers to the software that grants root-level access to the machine. Put the two together and you get 'rootkit', a program that gives someone - with legitimate or malicious intentions - privileged access to a computer.
There are four main types of rootkits: Kernel rootkits, User mode rootkits, Bootloader rootkits, Memory rootkits

Explanation Explanation Security Intelligence Sources ... Custom Block lists or feeds (or objects or groups) Block specific IP addresses, URLs, or domain names using a manually-created list or feed (for IP addresses, you can also use network objects or groups.) For example, if you become aware of malicious sites or addresses that are not yet blocked by a feed, add these sites to a custom Security Intelligence list and add this custom list to the Block list in the Security Intelligence tab of your access control policy. Reference: https://www.cisco.com/c/en/us/td/docs/security/firepower/623/configuration/guide/fpmc-configguide-v623/security_intelligence_blacklisting.html Explanation Security Intelligence Sources
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Custom Block lists or feeds (or objects or groups)
Block specific IP addresses, URLs, or domain names using a manually-created list or feed (for IP addresses, you can also use network objects or groups.) For example, if you become aware of malicious sites or addresses that are not yet blocked by a feed, add these sites to a custom Security Intelligence list and add this custom list to the Block list in the Security Intelligence tab of your access control policy.
Explanation Explanation Security Intelligence Sources ... Custom Block lists or feeds (or objects or groups) Block specific IP addresses, URLs, or domain names using a manually-created list or feed (for IP addresses, you can also use network objects or groups.) For example, if you become aware of malicious sites or addresses that are not yet blocked by a feed, add these sites to a custom Security Intelligence list and add this custom list to the Block list in the Security Intelligence tab of your access control policy. Reference: https://www.cisco.com/c/en/us/td/docs/security/firepower/623/configuration/guide/fpmc-configguide-v623/security_intelligence_blacklisting.html