The "FQDN Keep System" is a numeric field, so only numbers could be used for that.
If you specify the "FQDN Alternate" and the "FQDN Keep System" is not added or added with zero (0) value, then the codebase with the original system based hostname will not be generated. If you provide both keys and the "FQDN Keep System" has a value of any number greater than "0", then alongside with alternate hostnames, the system one will be generated.
When "FQDN Alternate" has a value and "FQDN Keep System" is zero - only the alternate hostnames from "FQDN Alternate" will be used in codebases.
If we speak about PS self-tests, then if "FQDN Alternate" is specified and "FQDN Keep System" is zero, then the first alternate name is used for self-tests.
Now, in regards to the Task Server portion in the Site Server, the Task Server (TS) publishes its URL to the Symantec Management Platform (SMP) using the internal Fully Qualified Domain Name (FQDN).
There is no specific code in the TS to send/receive Task Server FQDN. When we create a list of available Task Servers, we get FQDN names for TS's from Inv_AeX_AC_TCPIP database table using the fnTM_GetComputerResourceFQDN function.
To address that (only if the alternate name is just a change between the internal and external domain reference), create a new registry key, which will allow an alternate domain name used in the FQDN strings which will be used in the generation of HTTP & HTTPS codebases by Task Server when Basic Inventory is sent.
The "FQDN Replacement" is replacing only the Domain fields (see list below).
The current logic replaces next fields in the Basic Inventory:
For "AeX AC Identification" it replaces 'c2' field
For "AeX AC TCPIPv6" it replaces 'c8' field
For "AeX AC TCPIPv6" it replaces 'c4' field