forecasting in regARIMA for forward forecast

hi,

i was using a regression model with arima(1,1) errors for forecasting log GDP using CPI for the indian economy data. i'am using the exact code as in the matlab example given in the documentation:

load Data_USEconModel;

logGDP = log(DataTable.GDP);

dlogGDP = diff(logGDP); % For stationarity

dCPI = diff(DataTable.CPIAUCSL); % For stationarity

numObs = length(dlogGDP);

gdp = dlogGDP(1:end-15); % Estimation sample

cpi = dCPI(1:end-15);

T = length(gdp); % Effective sample size

frstHzn = T+1:numObs; % Forecast horizon

hoCPI = dCPI(frstHzn); % Holdout sample

dts = dates(2:end); % Date nummbers

Mdl = regARIMA('ARLags',1,'MALags',1);

EstMdl = estimate(Mdl,gdp,'X',cpi);

[gdpF,gdpMSE] = forecast(EstMdl,15,'Y0',gdp,'X0',cpi,'XF',hoCPI);

the program works nicely for the data if i have to check against a holdout sample. my question is now i do not want to check the forecast against the holdout sample but simple give me a 15 period forward prediction from the current last data. what will this line be now:

[gdpF,gdpMSE] = forecast(EstMdl,15,'Y0',gdp,'X0',cpi,'XF',hoCPI);

what will be 'Y0','X0' and 'XF'?

as the above line checks for the holdout sample i want a forward forecast of the gdp using current values of gdp and cpi?

your help would be much appreciated.