ENCs are available as Base Cells or Update Patches.
 Each ENC has an Update Application Date, an Issue Date and various Cleared Dates.
 
Base Cells are made available when each New Edition of an ENC is produced.
-The Update Application Date is the date that a Base Cell becomes available.
-In general, a Base Cell is produced when significant changes have been made to the ENC's coverage area, e.g., application of a hydrographic survey, channel alignment or shoreline modification.
-A Base Cell contains all the cartographic data within the coverage area of the ENC.
-The filename extension for a Base Cell is 000.
 
Update Patches can be made available many times during the edition life of an ENC.
-The Issue Date is the date that an Update Patch becomes available.
-In general, an Update Patch is produced when a critical change has been made to the ENC, e.g., application of a Notice to Mariner chart correction.
-An Update Patch only contains the cartographic data associated with the critical change, and must be applied to the Base Cell.
-The filename extension for an Update Patch is the number of the update, e.g., 001, 002, etc. These numbers are sequential and without omission.
 
Cleared Dates can be determined for each ENC.
-Clear Dates are based on the examination of Notice to Mariners published by NGA and USCG, and the application to the ENCs of critical corrections from these publications.
-If no critical corrections have been published since the ENC was last made available, then the ENC's Cleared Date is the date of the last examined Notice To Mariners publication.
-If there has been a critical correction published since the ENC was last made available, then the ENC's Cleared Date is the date of the last Notice To Mariners published prior to the publication of the critical correction.
-If the ENC is made available directly after the application of a critical correction, then the ENC's Cleared Date is the date of the Notice To Mariners publication that included the correction.