This package provides a queryparser, querybuilder and extra helper tools, to parse stored queries to actual results, used in new style collections. It includes a registry reader which reads operators, values and criteria from the Plone registry.
For each Plone release, its versions.cfg file at http://dist.plone.org/release/ pins a version of plone.app.querystring that works well with that Plone version. It is wise not to pick another version.
But for clarity, these are the correct relationships between the two versions:
On Plone 4.2 use 1.0.x.
On Plone 4.3 use 1.2.x.
On Plone 5.0 use 1.3.x.
Too new versions can cause problems. For example, the 1.1.x and 1.2.x series are intended for usage with Plone 4.3. They depend on plone.batching, which ships with Plone 4.3 but may cause problems_ with Plone 4.2.
.. _problems: https://dev.plone.org/ticket/12875