What is the difference between the SMM and preamble?

A preamble is an introductory statement or preliminary explanation as to the purpose of the document and the principles behind its philosophy. The term is particularly applied to the opening paragraph(s) of a statute, which recite historical facts which may be pertinent to the issue being discussed. It is often confused with the long title or the enacting formula of a law.
System Management Mode (SMM) is an operating mode first released with the Intel 386SL and available in later microprocessors in the x86 architecture, in which all normal execution (including the operating system) is suspended, and special separate software (usually firmware or a hardware-assisted debugger) is executed in high-privilege mode.