I have looked far and wide to see if I could piece this answer together myself from others who've asked for conditional drop-down selections, so please forgive me if this is redundant and answered elsewhere.
I'd simply like to create a condition in which a drop-down selection for Test Slot (CHOOSE, DM1, DM2, DM3, DR1, DR2, DR3, R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12, M1, M2, M3, M4, M5, M6) is limited to a subset of that if the list contains an item with Status=In Progress and Test Slot=DM1.
I'm using InfoPath 2013, SharePoint Designer 2013
Final Code, as advised by Joanna, within EditForm and NewForm.aspx
<script src="https://code.jquery.com/jquery-1.11.3.min.js" type="text/javascript"></script>
<script type="text/javascript">
$(document).ready( function() {
var dropdown = $(":input[title='Test Slot']");
if(checkItem("DM1")){
dropdown.find("option[value='DM1']").remove();
}
if(checkItem("DM2")){
dropdown.find("option[value='DM2']").remove();
}
if(checkItem("DM3")){
dropdown.find("option[value='DM3']").remove();
}
if(checkItem("DR1")){
dropdown.find("option[value='DR1']").remove();
}
if(checkItem("DR2")){
dropdown.find("option[value='DR2']").remove();
}
if(checkItem("DR3")){
dropdown.find("option[value='DR3']").remove();
}
if(checkItem("R1")){
dropdown.find("option[value='R1']").remove();
}
if(checkItem("R2")){
dropdown.find("option[value='R2']").remove();
}
if(checkItem("R3")){
dropdown.find("option[value='R3']").remove();
}
if(checkItem("R4")){
dropdown.find("option[value='R4']").remove();
}
if(checkItem("R5")){
dropdown.find("option[value='R5']").remove();
}
if(checkItem("R6")){
dropdown.find("option[value='R6']").remove();
}
if(checkItem("R7")){
dropdown.find("option[value='R7']").remove();
}
if(checkItem("R8")){
dropdown.find("option[value='R8']").remove();
}
if(checkItem("R9")){
dropdown.find("option[value='R9']").remove();
}
if(checkItem("R10")){
dropdown.find("option[value='R10']").remove();
}
if(checkItem("R11")){
dropdown.find("option[value='R11']").remove();
}
if(checkItem("R12")){
dropdown.find("option[value='R12']").remove();
}
if(checkItem("M1")){
dropdown.find("option[value='M1']").remove();
}
if(checkItem("M2")){
dropdown.find("option[value='M2']").remove();
}
if(checkItem("M3")){
dropdown.find("option[value='M3']").remove();
}
if(checkItem("M4")){
dropdown.find("option[value='M4']").remove();
}
if(checkItem("M5")){
dropdown.find("option[value='M5']").remove();
}
if(checkItem("M6")){
dropdown.find("option[value='M6']").remove();
}
});
function checkItem(option) {
var result = false;
var requestUri = "https://company.sharepoint.com/sites/EXT-sfants/_api/Web/Lists/getbytitle('700 MHz Provisioning List')/Items?$select=Status,Slot&$filter=( Status eq 'In Progress') and (Slot eq '"+option+"')";
$.ajax
({
url: requestUri,
type: "GET",
cache: true,
async: false,
headers:{
"ACCEPT": "application/json;odata=verbose"
},
success: function (data) {
if($(data.d.results).length !== 0)
result = true;
},
error: function () {
}
});
return result;
}
</script>