The seal is a marker the technique itself isn't the seal nothing in the data book or manga suggested the seal is anything more than a marker or the source of the technique. FTG is based on summoning jutsu a summon is brought to a place where a marking is made
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It isn't the marking that is the jutsu, the marking only specify's the location of the user nothing complex about it.
First you say people are more skilled at different things and then you come along and say FRS isn't nearly as complex as FTG? Your not making sense, if I was to believe your later comment then minato should have made FRS with ease. Both FRS and FTG are S-rank jutsu their difficulty is of the same rank.