The on-site upgrade enables you to add additional functionality to an in-circuit emulator when you need it - if necessary even just in time. This way you don't have to decide in advance which functionality you might need in the future.

Simply start with a basic model. More debugging power can be added within one working day via an on-site upgrade (same day service might be possible, however is not guaranteed). The cost of the upgrades is the price difference between the model used and the model wanted plus a handling fee.
What you are purchasing from Hitex with the MX51 or AX51 class is not anymore just an in-circuit emulator, it is also a 'debugging license'. The debugging license is implemented via different versions of firmware (the firmware is the software running on the microcontroller inside the in-circuit emulator). As the firmware is stored in a Flash memory, it can be modified and configured via the communication link to the PC.
If more debugging power is needed, an upgrade to a higher license can be ordered. The upgrade request will be fulfilled with an e-mail. It contains a program which can modify the firmware of the base unit to implement the upgrade.

Conclusion:
With the on-site upgrade engineers can choose an 8051 in-circuit emulator without the fear of having spent money for features which will not be used or - even worse - deciding for a system which is not powerful enough to cope with the challenges of the next project.