# What is a TRUE Union Word™?

In the spirit of the What is a Word™/Phrase™ series started by JLee, a special brand of Phrase™ and Word™ puzzles.
More specifically, this is an extended Word™ puzzle, built on the base of What Is a Union Word? by Confiers.

If a word/phrase conforms to a special set of rules, I call it a TRUE Union Word™. Use the examples below to find the rule.

$$% set Title text. (spaces around the text ARE important; do not remove.) % increase Pad value only if your entries are longer than the title bar. % \def\Pad{\P{2.0}} \def\Title{\textbf{ TRUE Union }} % \def\S#1#2{\Space{#1}{20px}{#2px}}\def\P#1{\V{#1em}}\def\V#1{\S{#1}{9}} \def\T{\Title\textbf{Words}^{\;\!™}\Pad}\def\NT{\Pad\textbf{Not}\T\ }\displaystyle \smash{\lower{29px}\bbox[yellow]{\phantom{\rlap{rubio.2019.05.15}\S{6px}{0} \begin{array}{cc}\Pad\T&\NT\\\end{array}}}}\atop\def\V#1{\S{#1}{5}} \begin{array}{|c|c|}\hline\Pad\T&\NT\\\hline % \text{ NO LABELS }&\text{ BALLOONS }\\ \hline \text{ PREMIXED SUMMER MIX }&\text{ I SLIMMED MIXUP EXAMS }\\ \hline \text{ FATHOMABLE AIR }&\text{ ABRAHAMITE FOB }\\ \hline \text{ JACKNIFE BINS }&\text{ BLUE SKIS FIJI }\\ \hline \text{ JEAN LEUILLIUX }&\text{ AXEL LJUNGDAHL }\\ \hline \text{ MWERA PEOPLE }&\text{ SWAMP PEOPLE }\\ \hline \text{ MENDELISM }&\text{ EMMA SLADE }\\ \hline \text{ NARCISSUS EEL }&\text{ ASSORTEDNESS }\\ \hline \text{ SLAG FILLS UP }&\text{ WALL OF GLASS }\\ \hline \text{ HOME ROOMS }&\text{ MOORSHOLM }\\ \hline \end{array}$$

And, if you want to analyze, here is a CSV version:

TRUE Union Words™,Not TRUE Union Words™
NO LABELS,BALLOONS
PREMIXED SUMMER MIX,I SLIMMED MIXUP EXAMS
FATHOMABLE AIR,ABRAHAMITE FOB
JACKNIFE BINS,BLUE SKIS FIJI
JEAN LEUILLIUX,AXEL LJUNGDAHL
MWERA PEOPLE,SWAMP PEOPLE
NARCISSUS EEL,ASSORTEDNESS
SLAG FILLS UP,WALL OF GLASS
HOME ROOMS,MOORSHOLM


The puzzle satisfies the series’ inbuilt assumption, that each word can be tested for whether it is a TRUE Union Word™ without relying on the other words.
These are not the only examples of TRUE Union Words™; many more exist.

What is the special set of rules these words conform to?

—HINTS—