Salesforce uses a 15/18 digits unique id to identify each record. These ids which we can see in the URL are 15 digit case sensitive id whereas the API returns an 18 digit case insensitive id.
The reason why we needed an 18 digit id was due to limitations in Excel VLOOKUP formulas and many legacy systems, which works only with case insensitive ids. The last 3 digits are checksum of the 15 digits ID
Understanding the conversion of 15 to 18 digit ID – Continue reading
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