command prompt - ZSH variable substitution in adam explanation -

I examined R TFM but could not understand it. The snippet is from <2> ZSH from Adam 2 prompt.

  local prompt_line_1a_width = $ {# $ {(%%) prompt_line_1a // (\% ([KF1] |) \ {* \} | \% [BBKF])}} < / Code>  
 I understand that the variable length is assigned to a local variable  prompt_line_1a  variable variable  
  $ {(S% %) Prompt_line_1a // (\% ([KF1] |) \ {* \} | \% [BBKF])}   
 
 
  < / Div> 
 
 "post-text" itemprop = "text"> 
 So, someone told me that I just have to look at the right place, there are two parts in the command:  < Pre>  $ {(flags) variablename}   
 and  
  $ {string // pattern / replacement}   
 Where  
    complete signal expansion (extension on the resulting word too)  < / Ul> 
 and for later  / replacements  can be removed if there is no replacement but should be outdated.