With Apache mod_php, php_admin_value can appear in the main server configuration but not in ordinary .htaccess files. PHP-FPM pools also support php_admin_value declarations using their own configuration syntax. The exact form depends on the server integration.
An administrator-enforced setting is stronger than a normal php_value declaration: scripts cannot override it through ini_set(), and lower-priority configuration cannot simply change it. That makes it useful for policies that application code should not be able to loosen.
The directive still applies only to the configured server or pool context. Setting it for one FPM pool does not automatically change another, and an Apache-specific declaration does not control a separate CLI runtime. The PHP directive being set must also be meaningful for the installed version.
Use it for intentional server-controlled settings, document the scope, check the effective value in a served request and verify changes after a controlled service reload or restart.