Difference between ⫾ (U+2AFE) and ⫿ (U+2AFF) in the context of Dijkstra's Guarded Command Language?
Non-deterministic guarded choice is an associative-commutative binary operator. My guess is that the character ⫾ U+2AFE is intended to be used as an infix binary operator and the character ⫿ U+2AFF, which is larger and described as “n-ary”, is intended to be used as a prefix operator which is typeset larger, in prefix position, and typically with a subscript and sometimes a superscript to indicate the range over which the operation is carried out.

$$⫿_{i=1}^n c_i \rightarrow a_i \qquad = \qquad  \mathsf{if}\; c_1 \rightarrow a_1 ⫾ \cdots ⫾ c_n \rightarrow a_n \;\mathsf{fi}$$

Typeset with inline math: $⫿_{i=1}^n c_i \rightarrow a_i = \mathsf{if}\; c_1 \rightarrow a_1 ⫾ \cdots ⫾ c_n \rightarrow a_n \;\mathsf{fi}$

It's the same relationship as between $+$ (binary) and $\sum$ (n-ary), or between $\times$ (binary) and $\prod$ (n-ary) ($\prod$ is sometimes a different product such as $\cdot$). For binary operators other than the two classics addition and multiplication, the n-ary version is the same glyph, just bigger.