We are looking for the meaning of prefix "EN" for Intel part 87C251SB16 or any documentation ?
This seems to be a very old microchip made by Intel, possibly discontinued already.
Usually information about this type of products is provided by a Field Application Engineer (FAE) under “Non Disclosure Agreement” between Intel and a 3rd party manufacturer or Intel Partner.
I found a datasheet in a 3rd party web site (not an Intel source) that shows the nomenclature used for this type of chips. You might want to check on that document to see if that information there answers your question. See page #4: