al RL

Spanish to English Science Chemistry; Chem Sci/Eng

On a manufacturing sheet (with spaces to fill out for operators) for a pharmaceutical. Context:
OPERACIÓN CONTROLES FECHA EMPIEZA TERMINA
1. Calentamiento a reflujo Tf: F:
2. Reflujo hasta disolución Tf: Disolución: F:
3. Enfriamiento a 70-75ºC Tf: F:
4. Filtración con FP-06 al RL Inspec: C.Placas: F:
5. Controles filtración Resultado: F: Frev:

I understand that FP-06 is grade 6 filter paper but I am puzzled at the "al RL". Does anyone know?

Thanks in advance!