Do you happen to know what the “X” calls out at the end of Altera part number EPF10K100EBC356-1X?

Hi,

Suffixes are optional letters at the end of a part number that specify certain device features. Sometimes it is used for Intel internal use only so you can ignore it, here 'X' indicates the 'PLLs in APEX™ and FLEX 10KE devices'.

Please refer the link below for more details,

https://www.intel.in/content/www/in/en/products/programmable/sample-ordering-codes.html

Regards,

Vicky