ADM1270 maximum ratings are listed below:
at UV, OV and FLB pin are limited to 6V, But if I look at the demo board schematic, when the input voltage is 60V, the UV pin is 6.9V.
Is this condition allowed?
Are there internal clamping diodes on pins?
If yes to which voltage are they referred?
Interesting question. It turns out that the OV pin here is programmed to shut down the system at 37.5V, so while 60V is within the spec for the ADM1270, this board is not configured to handle it by default. The EVAL-ADM1270 document is not clear on this point. The ADM1270 can safely operate at a maximum voltage of 60V, but you should set the OV threshold there, and not exceed the 66V ABS MAX spec.
To answer your specific questions:
1) No, 6.9V on the UV pin is not permitted under any circumstances.
2) There are internal ESD structures on the pins that will likely turn on if you apply excessive voltage, but this is not permitted.
3) The ESD structures are referred to GND. Do not rely on ESD to clamp at a particular (safe) voltage.