Answers of Kajukenbo and schily are incorrect
I have not enough "reputation" to "comment" on the answers, but I have to say that they are just wrong or, to be fair, misled by a POSIX specifications layout that was made for printers, not for browsers.
Misled by missing introduction / bad specs layout
The OP links correctly to the introduction of chapter 4: "Utilities" of POSIX' "Shell & Utilities" (= "XCU" part of POSIX) where a distinction between "mandatory" and "optional" utilities is given - see Gilles' explanation (as excellent as usual, but outdated for alias
as example, see below). Unfortunately in the POSIX 2004 specifications version there are no links to the rest of this chapter, and esp. not to the utilities itself. So eg. Kajukenbo obviously searched elsewhere and found the "utilities" index of the POSIX 2017 specifications - but only the index (see the "idx" in the url) and nothing else from XCU's chapter 4 to explain how to read this list.
The attempt to interpret the pure list of utilities leads to a "mandatory" list that mixes mandatory and optional utilities and creating an own (non-POSIX conform) category for "optional".
Proper specs navigation
Now we have the 2018 edition of POSIX 2017 with some improvements for browsing, so if you consult the XCU 2018 edition, you have "Previous" and "Next" in header and footer to navigate and a (badly named) "Home" that leads to the chapter's TOC. Unfortunately chapter 4 gives no simple list of all utilities, you have to consult the index link found by Kajukenbo. From there or following the order of chapter 4 you get to descriptions for each utility considered by POSIX. In these descriptions the decisive element is not the presence or absence of the word "DEVELOPMENT" that makes admin
non-mandatory (optional) and alias
mandatory, but the presence or absence of a "margin code", given as a superscript link in square brackets in the upper left corner of the SYNOPSIS paragraph like "[XSI]" for admin
. Note that esp. "XSI" is referred to in the POSIX specs under Conformance as "may support ... XSI", not "must".
The upper left corner Codes are introduced in the introduction of XCU's chapter 4 (see above) with a link to a code list that also has a section explaining this "Margin Code Notation". The "shading" of the SYNPOSIS paragraph mentioned there is usually rendered by some steel blue background, but this is by nature just a differentiating feature, not an identifying one, i.e. you may notice it only on switching between descriptions of mandatory and optional utilities, but not when directly accessing an optional one (and a blue link label on a steel blue background is not the best design idea).
POSIX versions
Note that since Gilles' answer alias
has changed from optional in POSIX 2004 to mandatory in POSIX 2008. So you have to take also in account the POSIX version against which compliance is claimed or searched. This might be confusing due to "standard" vs. "revision" vs. "edition" vs. "version". Here you best use the IEEE standard number as given in the header of specifications on opengroup.com, eg. the current "IEEE Std 1003.1-2017". The IEEE versioning is well documented in Wikipedia's POSIX entry, while the division in parts like XCU is well covered in Wikipedia's SUS entry which, on the other hand, sticks to POSIX "editions" instead of IEEE numbers.
Painful as it is, you might benefit by delving deeper into some UNIX and / or engineer's way of thinking.