Variables for accounting document item long text (OB56)?

Hello gurus

I have googled and searched on the forum but have not been able to find a proper answer.

We want to set up some default texts for FB01 acctg document item long texts.

We can see that you can create the texts in SPRO (trans OB56) and that there are sample SAP delivered texts with posting period variable $BUP:

AAAA     Rent for Walldorf storage building $BUP

LEAS     Leasing fee for vehicles in month $BUP

$BUP converts nicely to posting period and year - eg will insert 03/2015 but we don't know where the variable definition comes from.

We would like to do similar texts with a variable for year only (for example).

So.. where is variable $BUP defined and used?

Is there a list of available variables we can add to our line item text templates prefixed by $?

Many thanks for any input..