Get Cycle API help - expandos

I'm using the Get Cycle API call (https://zephyrsquad.docs.apiary.io/#reference/cycle/get-cycle/get-cycle).

According to the API docs:

"Get Cycle Returns a full representation of the Cycle for the given version. An Cycle JSON consists of the cycle details and an expandos driven executionSummaries for the cycle."

What does "expandos" mean? I've managed to gather that I can pass the request parameter "expand=executionSummaries" to see some details, but not everything I need. I'm hoping to be able to retrieve the list of tests (either issueIds or issueKeys) within the test cycle. Is there somewhere that better explains the values that users can pass in this request parameter?

Here's the response I'm currently getting back:
https://prod-api.zephyr4jiracloud.com/connect/public/rest/api/1.0/cycle/a36fcf4a-7306-4423-a66a-07f872fbb5c4?versionId=35633&projectId=17166&expand=executionSummaries

Response body:

{
"totalExecutionTime" : 0,
"folders" : { },
"createdByAccountId" : "557058:4d0b4c7d-d719-4e5c-97dc-9f74a6549565",
"totalLoggedTime" : 0,
"offset" : "0",
"totalExecutions" : 19,
"description" : "Created by ZAPI CLOUD API",
"cycleIndex" : "a36fcf4a-7306-4423-a66a-07f872fbb5c4",
"totalExecuted" : 0,
"creationDate" : "2021-12-20",
"projectCycleVersionIndex" : "17166_35633",
"executionsAwaitingLog" : 19,
"versionId" : 35633,
"totalDefects" : 0,
"createdBy" : "557058:4d0b4c7d-d719-4e5c-97dc-9f74a6549565",
"totalLoggedTimeFormatted" : "0m",
"name" : "2021-12-2009:48:49 Test Cycle -- API DEMO",
"totalExecutionTimeFormatted" : "0m",
"action" : "collapse",
"totalFolders" : 0,
"id" : "a36fcf4a-7306-4423-a66a-07f872fbb5c4",
"projectId" : 17166,
"startDate" : "2021-12-20",
"executionSummaries" : [ {
"executionStatusKey" : -1,
"count" : 19,
"executionStatusName" : "UNEXECUTED",
"executionStatusDescription" : "The test has not yet been executed.",
"executionStatusColor" : "#A0A0A0"
} ]
}