'What is the meaning of vertical bar in array dtype ('|S58')

I have an error when I tried to do

np.savetxt(f, te_filename)

It returns an error of

% (str(X.dtype), format))

TypeError: Mismatch between array dtype ('|S58') and format specifier ('%.18e')

I checked the doc page,

Seems that S stands for string and 58 is the length, what is the meaning of vertical bar then?

python numpy

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