Standex Electronic's Photo-MOSFET Solid State Relay part number system is similar to their system for Reed Relays. The first three digits are the product type, where SMP stands for Standex-Meder Photo-MOSFET Solid State Relay. The fourth digit is the number of “poles” or switching elements: one or two. The fifth digit is the contact form: Normally Open (A), Normally Closed (B), or Changeover (C). The sixth and seventh digits are the series number each with its own unique specifications. The eighth digit represents the number of external pins on the package: 4, 6 or 8. The ninth digit is the package type: DIP, SMD or SOP. The tenth digit indicates parts shipped in Tubes (T) or in Tape & Reel (R). The eleventh digit, if used, denotes special features such as Reinforced Isolation or AEC-Q101/Hi-Temp. Not all combinations are available, so please consult the factory with any questions. Part number marking on the part itself consists of the fourth through ninth digits of the part number due to space limitations.