Here's a sample SIP patron information response (patron info obfuscated):

64YYYY      YY  00020190214    092541000000010001001700000000AOCC|AA12345|AETest User|BLY|CQY|BV81.51|CC20.00|AV $5.00 "F" The twentieth century : 03/02/2018 23:59|AV $4.75 "F" The twentieth century : 04/20/2018 23:59|AV $2.50 "F" Victoria. 04/20/2018 23:59|AV $2.50 "F" Before the flood 04/20/2018 23:59|AV $2.50 "F" Scandal. 04/20/2018 23:59|AV $3.25 "F" 25 04/27/2018 23:59|AV $3.25 "F" WOW gospel. 04/27/2018 23:59|AV $3.25 "F" When was the last time 04/27/2018 23:59|AV $0.26 "F" You want it darker 04/27/2018 23:59|AV $1.00 "F" Master of none. 04/27/2018 23:59|AV $1.25 "F" Elizabeth I : 05/04/2018 23:59|AV $1.50 "F" My wars are laid away in books : 05/04/2018 23:59|AV $1.50 "F" The female persuasion 05/04/2018 23:59|AV $4.00 "F" 25 06/22/2018 23:59|AV $5.00 "FU" My wars are laid away in books : 06/22/2018 23:59|AV $5.00 "PF" Lost Item My wars are laid away in books : 33550008782919|AV $35.00 "L" Lost Item My wars are laid away in books : 33550008782919|AV $ "" |BD123 Main St, Anytown KS, 66000|testuser@example.com|BF321-555-5555|PCADULT|PIN|AFGreetings from Koha.  -- Patron owes 81.51|AY0AZD6F8

The screen messages from the patron fines are formatted like this:

AV $4.75 "F" The twentieth century : 04/20/2018 23:59

Where "F" is the accounttype from accountlines. This should display as the human readable 'Overdue fine'.

The list of these messages is hard coded in Koha, and is documented here:

https://koha-community.org/manual/18.05/en/html/faq.html#fines-table

The accounttype 'FU' is particularly problematic when it shows on user displays, because it's a common abbreviation for an obscenity in English.