You may ask that this Pipeline build wait for completion of the downstream build. In that case the return value of the step is an object on which you can obtain the following read-only properties: so you can inspect its result and so on.

number
build number (integer)
result
typically SUCCESS, UNSTABLE, or FAILED (may be null for an ongoing build)
displayName
normally #123 but sometimes set to, e.g., an SCM commit identifier
description
additional information about the build
id
normally number as a string
timeInMillis
time since the epoch when the build was scheduled
startTimeInMillis
time since the epoch when the build started running
duration
duration of the build in milliseconds
previousBuild
another similar object, or null
nextBuild
similarly
absoluteUrl
URL of build index page
buildVariables
for a non-Pipeline downstream build, offers access to a map of defined build variables; for a Pipeline downstream build, any variables set globally on env
changeSets
a list of changesets coming from distinct SCM checkouts; each has a kind and is a list of commits; each commit has a commitId, timestamp, msg, author, and affectedFiles each of which has an editType and path; easiest to use this inside a method marked @NonCPS to avoid storing intermediate values
rawBuild
a hudson.model.Run with further APIs, only for administrator-approved scripts outside the sandbox; you will generally need to use this inside a method marked @NonCPS to avoid storing intermediate values

If you do not wait, this step succeeds so long as the downstream build can be added to the queue (it will not even have been started). In that case there is currently no return value.