Variable substitution in Automation Object parameters
Regular processing
During test execution, when test configration contained in Automation Objects is loaded, all loaded additional type parameters (e.g. HTTP Headers) are pre-processed to load configured variable names. Given a Variable XXX is configured in Automation Object with value 123, the Header entry Value={XXX} will be read as Value=123 thus allowing for dynamic header modification in processing.
There is a limitation of 250 characters to such an expanded value.
Variables named with underscore at the beginning and the end, like _XXX_ will not be processed at this stage
Special processing for URL substitution
In specific scenarios, the URL itself might be constructed from a variable and require further variable substitution - specifically if the first substitution would cross the 250 character limit. To handle such scenarios, the above rule for variable named with underscore at the beginning and the end, like _XXX_ was introduced.
Variables with underscore (e.g. _XXX_) will be substituted in the URL processing later on.
This allows to construct an URL first with a variable like URLCONST having a value like query={_Q_}¶m={_P_}, and then fill in the query and parameters with values from variables _Q_ and _P_ respectively. This way, the complete URL can be longer than the 250 character limit for substitution in additional parameters.