A SQL snippet can optionally have parameters. To define snippet parameters, specify them in the "Params" field and use them in the snippet definition with square bracket notation (i.e. just like when the parameter is a snippet). To use a parameterised snippet in a report, pass it the parameters in brackets (like a function call):
Any parameters that are defined on a snippet but not passed will have a "NULL" value.
An example is the following currency conversion snippet:
Usage in a report:
SUM([in_usd(account_revenue,account_currency)]) as revenue_in_usd