Each record in Salesforce has a unique value known as a record ID. Salesforce automatically assigns a unique Id to each Contact, Account, and other objects. Two new IDs are established in the background, one with 15 digits and the other with 18.
Difference between 15 and 18 Digit Ids
Salesforce internally uses case-sensitive 15-digit Ids to manage its database. Because they are case-sensitive, Object A can have the Id 003000500000abc and Object B can have the Id 003000500000ABC and these are seen as Unique from a Salesforce perspective.
On the other side, 18 Digit Ids are not case-sensitive and unique e.g. Object A could have the Id 003000500000ABCCBA and Object B could have the Id 003000500000CBAABC.
A checksum of the capitalization of the first 15 characters is contained in the last three digits of the 18-digit identification number, this Id length was developed as a workaround for legacy systems incompatible with case-sensitive Ids. The 18-digit ID is referenced through the APIs. You will need this while using the tools like data loader and workbench. The 18-digit Id is always returned by the API even though the 15-digit Id can be used as input.
Because the 18-digit IDs are case-insensitive, it is best practice for salesforce to use them for APIs, formulas, or as references.
Why are two separate record IDs required?
The case sensitivity of these two holds the key to the solution. The use of a case-sensitive ID can result in duplication in numerous systems. Case-sensitive IDs, for instance, are not recognized by programs like Access and Excel. In this case, the value will be hosted by both capital and tiny letters of A. A discrepancy in record IDs may result from this.
The 18-digit code is utilized to find the solution to this issue. Access to all API activity types also requires using these case-safe values. These 18-digit IDs are created by appending a suffix to the 15-digit ID that already exists.