This trigger function works correctly:
{vm.memory.size[pavailable].max({$TESTVALUE})}<10

And this {vm.memory.size[pavailable].max({$TESTVALUE}m)}<10
get this error:
Cannot implode expression "{<MyHost>:vm.memory.size[pavailable].max({$TESTVALUE}m)}<10". Incorrect trigger function "max({$TESTVALUE}m)" provided in expression. Invalid first parameter. [triggers.php:185 ? CFrontendApiWrapper->update() ? CApiWrapper->__call() ? CFrontendApiWrapper->callMethod() ? CApiWrapper->callMethod() ? CFrontendApiWrapper->callClientMethod() ? CLocalApiClient->callMethod() ? call_user_func_array() ? CTrigger->update() ? CTrigger->updateReal() ? CApiService::exception() in /usr/share/zabbix/include/classes/api/services/CTrigger.php:1203]
CONFIGURATION OF TRIGGERS

It is not problem for functionality, because literal "m" can be in macros; but it is not comfortably for writing trigger name. ("RAM free (17.52%) lower 20% at last 3m minutes")