The principal sources of systematic error in the peak locations in powder diffraction patterns are:
The external standard method establishes a $\Delta2\theta$ vs. 2$\theta$ calibration curve for the diffractometer by measuring characterized lattice parameters of one or more samples. The peak finding algorithm will routinely apply this calibration curve to correct the peak 2$\theta$. This procedure removes all but the sample transparency error, which usually is the largest correction required.
The commonly used internal standard method will eliminate the transparency error, but requires a separate analysis run in which an internal standard like Silicon has been added to the specimen. A monolayer specimen mounting technique on a zero background holder (ZBH) allows the external standard procedure to give results equivalent to the internal standard.
In addition to the convenience of the fully automated application of this external standard ZBH method, it avoids a common problem of the internal standard method: extrapolation of the calibration curve to low angles is an inherently dangerous (and incorrect) procedure commonly practiced in the internal standard method. The ZBH method avoids this permitting the combining of multiple standards from materials with small lattice parameters like Si to large values like Silver Behenate, whose c axis is 58.38\AA.