This issue was raised on https://lists.oasis-open.org/archives/office-comment/201502/msg00009.html
cell-content-is-between is defined in 19.468 style:condition of ODF 1.2 part 1.
Currently cell-content-is-between appears under specified since the current definition simply says "cell-content-is-between(value1, value2): true if the style is applied to a table cell whose value is between value1 and value2."
The term `is-between' could include equality with value1 and/or value2 or not.
value1 and/or value2 can be a numberValue, a string or an expression. It is unspecified how these different types are handled. It is also not clear what happens when expressions used here do not do not return a numberValue or string. The initial reporter on the comment list was specifically worried about the expression being a reference to an empty cell.
Clearly cell-content-is-not-between also requires clarification.