What is SMS field in Contacts?

[x21]

When I imported my contacts to my x51v (a month ago- nothing to do with the A06 stuff), everything went fine, but each record with a mobile phone# had a field added that was not there before "SMS" with the same mobile phone# as the "mobile phone" field. Also, if I add a record and it includes a mobile number, an SMS entry is added automatically with that same number. What is this, why is it being created automatically, and what's it used for? Can I just delete them? Thanks.
jim

[AndrewZ]

"mobile phone" number is used to make phone calls
SMS number is used to send SMS (if you have software/hardware to do it)

if you don't know what is the SMS at all - http://en.wikipedia.org/wiki/Short_message_service