LandXML
Short answer
An open XML schema for exchanging civil engineering and survey data — surfaces, alignments, parcels — between Civil 3D and other software.
LandXML is the open exchange format used in civil engineering and land surveying to move design data between applications. A LandXML file can describe a TIN surface, alignments with stationing, parcels, pipe networks, and survey point groups. Civil 3D imports/exports LandXML natively, and almost every surveying and machine-control system reads it. Plugins that integrate with field data, third-party hydraulics, or earthworks software typically round-trip through LandXML.