A partial index with a filter {a: {$gt: 5}} should be able to answer the following queries

{a: {$in: [5.1, 6]}}
{a: {$in: [7, 8, 9]}}

but not

{a: {$in: [5]}}
{a: {$in: [3, 4.9]}}
{a: {$in: [null]}}

because each element in the $in array is greater than the value of the ComparisonMatchExpression. This applies to partial index filters involving $eq, $gt, $gte, $lt, and $lte.