There are no attributes recognized as "unique id" or "client id" (more on client id in a second). All document IDs need to be unique or you'll have a helluva time trying to get the page to compile/run nicely.
Client ID comes into play with the new ID generation features that allow for more predictable, developer-friendly control/form element names, and is expressed as "client id mode". Scott Guthrie has this great post featuring this on
his blog[
^].
If there's something else (or a link) you're referring to, please post and I'll try to help out.
Cheers.
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