i: They're set at the initialization of an instrument (init pass).
k: control variables. Updated each control cycle
a: audio variables. Unlike control variables, they are a vector whose size is ksmps. Usually, they store audio samples. Updated each control cycle
p: score parameter fields
S: string variables downsamp, by default, takes the first value of an a-variable array and assigns it to a k-variable. With the iwlen parameter, a length of the sub array can be defined of which the avarage is taken. max_k finds the max or min value in the a-array. rms samphold avar = kvar: all elements in avar are set to kvar's value. upsamp is equivalent, but less efficient. interp which is equivalent to avar=a(kvar). kval = ival kvar might not be initized at the init pass. vaget and vaset print opcode.