Usually on a new project is is standard practice to analyse the full length of every drill hole so the geology staff can become familair with which rock types and which alteraction types have the potential to host mineralization.  Assay intervals are usually based on geological,  alteration or mineralization and usually there is a maximum length, say 2.0 m. Usually the core is split or cut in half for each interval with one half submitted for assay and the other kept for reference of future work - metallurgical or ARD test work etc.

In some cases once a particular geological unit has been determined to be non-mineralized geologists may decide not to sample it in order to save on assay costs and the full core is kept for fututre reference. Similarly once a particular unnit or alteration or mineralizatrion style has been determined to have economic potential it is usually always sampled.