CSS display defines inline-level as

inline-level: Content that participates in inline layout. Specifically, inline-level boxes and text.

First, the "specifically, inline-level boxes" part seems completely superfluous.

Moreover, if I understood properly, text is not inline-level. Instead, what happens is that text directly contained inside a block container is wrapped inside an anonymous inline element, and it's that element what is inline-level, not the text itself.

And then, I guess "participates in inline layout" means "participates in an inline formatting context". The former expression does not seem to be used anywhere else, while the latter is used everywhere. This is confusing.

And similarly for block-level:

block-level: Content that participates in block layout. Specifically, block-level boxes.

Same problem about "block layout", and "specifically, inline-level boxes" seems superfluous too.

Instead, I would just say:

inline-level: Content that participates in an inline formatting context.
block-level: Content that participates in a block formatting context.

While we are at it, why not add

flex-level: Content that participates in a flex formatting context. [CSS3-FLEXBOX]
grid-level: Content that participates in a grid formatting context. [CSS3-GRID-LAYOUT]