Improvement Location Certificates, or (ILC's) are used to locate the improvements on a parcel of property. Typically mortgage or title companies require them for the buying and selling of homes. They can also be used by municipalities for construction permitting.
ILC's are certified by a licensed surveyor for the benefit of mortgage companies and title insurance companies, but are not surveys in the strict sense of the word. Before a mortgage or title insurance is issued, the interested parties want to have some assurance that the improvements on the subject property do not encroach onto adjoining properties, and that neighboring improvements do not encroach onto the subject property. In most cases an Improvement Location Certificate will be sufficient to show the relationship (location) of the structures (improvements) to the deed lines as described in the legal description. This information can show potential encroachments onto other properties or into areas reserved for easements.
Keep in mind, an ILC is a certificate, not a survey! The measurements shown on an ILC are to the approximate record position of the property line. This line is subject to change if a full boundary survey is performed.