As in āhvāna आह्वान - Invocation
火字門，入諸法喚不可得故
Kumarajiva
入嗑縛字門，悟一切法可呼召性不可得故
Xuanzang
HVA because true appellations (āhvāna).
Conze
The syllable HVA [means] all mental phenomena are openings because of the non-recognition of the invocation (āhvāna).
Jayarava