The resource number comes from a 3D geomodel  based on the drillholes. To calculate the PEA economics they have to define a reasonable pit shell design that would allow extraction from an open pit. The geometry of most ore bodies prevents all of the resource to fit within the pit design. Typically you expect to get 60-70% of the resource within the pit. The rest is still there but would have to be extracted by some other mining method such as block and cave, if the grade is high enough. Here's an example from SC that shows the pit shell outline and high grade material below it, which would not be counted as recoverable metal in a PEA or FS.