HP E4418B Programming Manual page 120

Power Meter Remote Operation
An Introduction to the SCPI Language
<character response data> Definition
Throughout this chapter <character_data> is used to represent
character data, that is, A - Z, a - z, 0 - 9 and _ (underscore). For example:
START and R6_5F. The format is defined as:
<upper-case
alpha>
<upper-case
alpha>
<digit>
<NAN> Definition
Not a number (NAN) is represented as 9.91 E37. Not a number is defined in
IEEE 754.
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