According to SCPI each command header can have short and long form. In order to distinguish both forms in command syntax description, upper and lower case characters are used. Note, that SCPI is case-insensitive, the usage of upper and lower case characters is done solely for the purpose of syntax definition. You may even mix upper and lower case. There is no semantic difference between upper and lower case in program messages. Same applies for parameters - they may have short and long forms.
Example 1. Let’s consider such syntax description: SYSTem:CONFigure <parameters>
Here SYST and CONF specify the short form, and SYSTem and CONFigure specify the long form. It means that the instrument will accept the following command headers: syst:conf , SYST:CONF , SYSTEM:CONFIGURE, SYST:CONFIGURE, or even sYstEm:cOnFiGuRE. However, SYSTE or CONFIG headers are not allowed and will cause a command error, because they don’t refer to short or long forms.
Example 2. Let’s consider such syntax description :FORMat[:DATA] <format> , where <format> is one of ASCii, REAL or PACKed. Such argument syntax indicates that the argument may be specified as: ASCII, ASC, asc, REAL, real, PACKED, pack, PACK.