Is FileMaker Pro Advanced required?
FileMaker Pro Advanced is only required to copy/paste the Custom Functions that fmAccounting Link (Xero Edition) uses when you are integrating this into your own FileMaker solution files. Otherwise you can use both FileMaker Pro and Pro Advanced when using the fmAccounting Link (Xero Edition) functionality.
N.B. since the release of FileMaker Pro v17 there are no longer 2 separate applications (FileMaker Pro and FileMaker Pro Advanced). FileMaker Pro v17 or later now includes the advanced tools found previously in the FileMaker Pro Advanced Application - you can can enable these in the FileMaker Pro preferences.