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And what the use of this field?
Thank you.

I am no native english speaker and also a little bit confused.
AFAIK ETA is the abbreviation for Estimated Time of Arrival
I think this field can also be used to estimate the efforts to resolve an issue.
I hope other users will also give comments for this.

You are able to configure the available values by changing the following enumeration in your config_inc.php
$g_eta_enum_string= '10:none,20:< 1 day,30:2-3 days,40:< 1 week,50:< 1 month,60:> 1 month';

As for the meaning, you're correct that it stands for Estimated/Expected Time of Arrival.