Response time: estimated maximum time to get the final result code for the AT command execution. It is the time needed to provide the response in the worst case, e.g. when all the steps
that have to be run to carry out the operation take the longest time to be performed; it is based on a theoretical estimation, derived by the the 3GPP specifications in case of AT commands related to cellular modem features (e.g. registration, de-registration, PDP context activation). For additional details on the response time of cellular network related AT command, see Maximum vs typical response time of cellular network related AT commands.
More precisely, the response time considers the time from the complete acquisition of the command line to the issuing of the command result code.
This kind of response time is generally lower than the time measured by the application on the DTE, because the issuing of the command on the DTE
is influenced by the AT interface characteristics (e.g. the synchronous/asynchronous transfer type, the selected baud rate, etc.), by power saving and flow control,
which introduce a variable latency in the command acquisition by the DCE.
For example, the maximum expected response time shall be extended if the communication with the module is carried out on a MUX virtual port, because in this case the command line and the result code are transferred via a specific protocol running on the physical port, that might introduce additional communication delay due to framing and re-transmissions.
Similarly, the maximum expected response time of AT commands accessing the SIM shall be extended if the module is using a remote SIM card via SAP instead of the local SIM card.
If the response time for a command is left blank (actually "-"), it is an "immediate" response. It means that the command is executed without asynchronous requests to the protocol stack or the internal applications, which usually require time to be answered: the command execution is synchronous (implying that no long blocking processing is done) and lasts a negligible time (the command response is issued by the module in typically less than 10 ms, and in any case less than 1 s).
The response time shall be extended if the issued AT command triggers a service that cannot be served immediately due to concurrent access to the same service or resource via AT commands issued on a different communication port or from internal applications; typical examples are registration commands and SIM access, that can be also autonomously triggered by the module (e.g. auto-COPS) and can therefore postpone the execution of the AT commands issued by the user.