The first octal digit combines setuid 4, setgid 2, and sticky 1. These bits do not add extra characters to the nine-character display. Instead, setuid replaces the owner's execute position, setgid the group's, and sticky the others' position, while letter case preserves whether execute is also set.
Lowercase means execute is also set
4755 displays rwsr-xr-x because setuid and owner execute are both set. 2750 displays rwxr-s--- because setgid and group execute are set. 1777 displays rwxrwxrwt because sticky and others execute are set. The letter marks two bits at once; it is not a fourth permission in the triplet.
Uppercase means the paired execute bit is absent
4644 displays rwSr--r--: setuid is set but owner execute is not. 2640 displays rw-r-S---: setgid is set without group execute. 1700 displays rwx-----T: sticky is set without others execute. Uppercase S or T must never be normalized to lowercase because that would add an execute bit.
Special-bit effects depend on object and system
On executable files, setuid and setgid can affect effective IDs when the platform and policy honor them. On directories, setgid commonly controls group inheritance; sticky commonly restricts who may remove or rename entries in a writable directory. Filesystems, privileges, mounts, and system policy can ignore or clear bits, so ChmodCalc explains the modeled mode but cannot predict runtime security.