Let $n$ and $d$ represent numerator and denominator respectively, where $n \le d$. Similarly, $s$ and $e$ represent startAmount and endAmount. Assume that $s \neq e$ and for the sake of simplicity lets assume that $n$ and $d$ are in reduced form (coprime, i.e., $\operatorname{gcd}(n, d) = 1$).
Then, the following conditions have to be true for an order (here $a \mid b$ means $a$ divides $b$):
This severely limits the possibilities of an order. For example, if $\operatorname{gcd}(s, e) = 1$, then a strict partial order is impossibleonly a full fill (1 / 1) would ever get past such checks.
Context: Seaport.sol
Alice places a partial order with startAmount = 1000 and endAmount = 2001. Because 1000 and 2001 are coprime, such an order can only be fully filled.
Partially fillable orders with gcd(startAmount, endAmount) == 1 should ideally be disallowed. This may be done at the frontend to simplify the code.
