UT and RecID in ISI Web of Science

[T Van]

Hi everyone,

I have a question about ISI Web of Science. In  ISI WoS, each article is 
represented by an identifier called UT, however there's also another 
identifier called RecID. What are differences between them? (I only know 
that RecID is could be used to identify cited references of a given 
article). Are they unique (i.e. with an article there is only 1 UT 
and/or 1 RecID)?

Thank you in advance,
T. Van