Ipposnif variables can be used in sql expressions and sql assertions of the strategies.
Some of these variables can be used directly.
For example @is_in_play contains the in play status of the race and can be used to limit the betting to a certain status of the race:
<place condition="@is_in_play=1" size="2" price="1.01">

Other variables are tipically used as parameters passed to functions.
For example @id_race, @id_horse or @id_strategy are normally used to retrieve results from functions:
<place condition="dbo.fn_raceproperty(@id_race,'overround_back') < 110" size="2" price="1.01">

If the variable you need is missing, read in the Customize page how to add a new one.