If we have a tensor \begin{equation*} T_{abc}^d \end{equation*} What does the following, brackets containing lines, notation mean? \begin{equation*} T_{[a|b|c]}^d \end{equation*} This type of notation appears in Reference 1, see (3.3) (b).
Reference 1,
HLAVATÝ, VÁCLAV. “The Holonomy Group I. The Curvature Tensor.” Journal of Mathematics and Mechanics, vol. 8, no. 2, 1959, pp. 285–307. JSTOR, https://www.jstor.org/stable/24900561 . Accessed 22 Aug. 2022.
This is the first of six papers on ‘The Holonomy Group’, by V.Hlavaty, that are all available on JSTOR. Individuals may be able to sign up for a free account with JSTOR.
Other Information
I am aware of notations that use square brackets or parentheses as follows \begin{align*} T_{[ab]c}^d &= \frac{1}{2} \left(T_{abc}^d - T_{bac}^d \right) \\ T_{(ab)c}^d &= \frac{1}{2} \left(T_{abc}^d + T_{bac}^d \right) \end{align*} The following questions may be of interest to the reader.
https://physics.stackexchange.com/questions/95133/bracket-notation-on-tensor-indices