can someone explain what the SEID is please?

Chalk ~ SEID is an acronym for Secure Element Identifier.

"The Secure Element chip. This is an NFC chip that contains data such as the SEID (Secure Element Identifier) for secure transactions. This chip is commonly found in smartphones and other NFC enabled devices." [Source: Wikipedia]

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See the following from http://www.dummies.com/consumer-electronics/smartphones/iphone/your-iphones-abou t-feature/

SEID: This setting refers to chipset identity that is related to security and NFC, the mechanism used as part of the Apple Pay mobile payments system on the iPhone 6 and 6 Plus.

Ignore the "dummies" source -- I didn't know either. Also, it is a bit out of date as it also applies to iPhones newer than the 6.