lpslot selects the slot to be use by further lp opcodes. This is the way to load and reference several analyses at the same time.
Here is a typical orc using the opcodes:
ipower init 50000 ; Define sound generator ifreq init 440 asrc buzz ipower,ifreq,10,1 ktime line 0,p3,p3 ; Define time lin lpslot 0 ; Read square data poles krmsr,krmso,kerr,kcps lpread ktime,"square.pol" lpslot 1 ; Read triangle data poles krmsr,krmso,kerr,kcps lpread ktime,"triangle.pol" kmix line 0,p3,1 ; Compute result of mixing lpinterp 0,1,kmix ; and balance power ares lpreson asrc aout balance ares,asrc out aout