This notation is in the form a [ ↑ ↑ b ↑ ] c {\displaystyle a[\uparrow\boldsymbol{\uparrow}^b\uparrow] c} or a [ ↑ ↑ b ↑ ] c d {\displaystyle a[\uparrow\boldsymbol{\uparrow}^b\uparrow] ^cd}
I define it recursively as follows:
a [ ↑ ↑ 0 ↑ ] b {\displaystyle a[\uparrow\boldsymbol{\uparrow}^{0}\uparrow] b} = a b {\displaystyle a^b}
a [ ↑ ↑ b + 1 ↑ ] 0 {\displaystyle a[\uparrow\boldsymbol{\uparrow}^{b+1}\uparrow] 0} = a [ ↑ ↑ b ↑ ] a {\displaystyle a[\uparrow\boldsymbol{\uparrow}^{b}\uparrow] a}
a [ ↑ ↑ b ↑ ] 1 c {\displaystyle a[\uparrow\boldsymbol{\uparrow}^b\uparrow] ^1c} = a [ ↑ ↑ b ↑ ] c {\displaystyle a[\uparrow\boldsymbol{\uparrow}^b\uparrow] c}
a [ ↑ ↑ b ↑ ] c + 1 d {\displaystyle a[\uparrow\boldsymbol{\uparrow}^b\uparrow] ^{c+1}d} = a [ ↑ ↑ b ↑ ] ( a [ ↑ ↑ b ↑ ] c d ) {\displaystyle a[\uparrow\boldsymbol{\uparrow}^b\uparrow] (a[\uparrow\boldsymbol{\uparrow}^b\uparrow]^cd)}
a [ ↑ ↑ b + 1 ↑ ] ( c + 1 ) {\displaystyle a[\uparrow\boldsymbol{\uparrow}^{b+1}\uparrow] (c+1)} = a [ ↑ ↑ b ↑ ] a [ ↑ ↑ b + 1 ↑ ] c a {\displaystyle a[\uparrow\boldsymbol{\uparrow}^{b}\uparrow] ^{a[\uparrow\boldsymbol{\uparrow}^{b+1}\uparrow] c}a}
a [ ↑ ↑ ] b {\displaystyle a[\uparrow^\uparrow]b}
Maybe unfinished