link that explain ICU_LOCALE locale concept about local variant code and keywords.

DROP DATABASE IF EXISTS dbicu;
CREATE DATABASE dbicu LOCALE_PROVIDER icu LOCALE 'en_US' ICU_LOCALE 'en-u-kf-upper' template 'template0';
\c dbicu
CREATE COLLATION upperfirst (provider = icu, locale = 'en-u-kf-upper');
CREATE TABLE icu(def text, en text COLLATE "en_US", upfirst text COLLATE upperfirst);
INSERT INTO icu VALUES ('a', 'a', 'a'), ('b','b','b'), ('A','A','A'), ('B','B','B');
SELECT def AS def FROM icu ORDER BY def;
SELECT def AS en FROM icu ORDER BY en;
SELECT def AS upfirst FROM icu ORDER BY upfirst;
SELECT def AS upfirst_explicit FROM icu ORDER BY en COLLATE upperfirst;
SELECT def AS en_x_explicit FROM icu ORDER BY def COLLATE "en-x-icu";