If there are both (1) a plugin variable function and (2) a user-defined function with the same name, WHICH will identify the plugin one but TCC will use the user-defined one.
Code:
v:\> which @icmp
@icmp is a plugin variable (SYSUTILS)
v:\> echo %@icmp[jpsoft.com]
36
v:\> function icmp foo
v:\> which @icmp
@icmp is a plugin variable (SYSUTILS)
v:\> echo %@icmp[jpsoft.com]
foo