The best description of the organization's procedures in this context is that they represent a cause of the organization's accumulation of large travel advances. The lack of a requirement for justification for travel advances greater than a specific amount is a procedural gap that directly contributes to the accumulation of large travel advances. This gap in the procedure is the root cause that leads to the observed condition of large travel advances accumulating without sufficient oversight or justification.
Reference: IIA's International Standards for the Professional Practice of Internal Auditing, Standard 2310 - Identifying Information, and related practice advisories on root cause analysis in audit observations.