All the FormalSeries involved should have the same coefficient ring. The function substitutes the variables in s with the series in the BasicList l. Also note that this also works when s is a FormalGroupLaw which is a subclass of FormalSeries.
i1 : R=ZZ[x,y]
o1 = R
o1 : PolynomialRing
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i2 : s = series(x^2+x+y,2)
2
o2 = FormalSeries{x + x + y, 2}
o2 : FormalSeries
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i3 : substitute(s,{s,s})
2 2
o3 = FormalSeries{3x + 2x*y + y + 2x + 2y, 2}
o3 : FormalSeries
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