i1 : X = PP_QQ^(1,3);
o1 : ProjectiveVariety, curve in PP^3

i2 : a = 4, b = 2;

i3 : phi = rationalMap X;
o3 : MultirationalMap (rational map from PP^3 to PP^2)

i4 : assert(phi <==> multirationalMap {rationalMap ideal X})

i5 : phi = rationalMap(X,a);
o5 : MultirationalMap (rational map from PP^3 to PP^21)

i6 : assert(phi <==> multirationalMap {rationalMap(ideal X,a)})

i7 : phi = rationalMap(X,a,b);
o7 : MultirationalMap (rational map from PP^3 to PP^5)

i8 : assert(phi <==> multirationalMap {rationalMap(ideal X,a,b)})

If you want to consider $X$ as a subvariety of another multiprojective variety $Y$, you may use the command X_Y. For instance, rationalMap(X_Y,a) returns the rational map from $Y$ defined by a basis of the linear system $H^0(Y,\mathcal{I}_{X\subseteq Y}(a))$ (basically, this is equivalent to trim((rationalMap(X,a))Y)).
i9 : Y = random(3,X);
o9 : ProjectiveVariety, surface in PP^3

i10 : rationalMap(X_Y,a);
o10 : MultirationalMap (rational map from Y to PP^17)

i11 : rationalMap X_Y;
o11 : MultirationalMap (rational map from Y to PP^2)
