jstartup/release is a SAP Parameter attribute which is used to control Version string for backward compatibility 640 to 620 information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
JControl in versions 6.40 and 6.30 is backward-compatible with version
6.20. However, this parameter must be set to the value "jstar
tup/release=620" for this.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.