This code system http://hl7.org/fhir/substance-amount-type defines the following codes:

CodeDisplay
Average Average
Approximately Approximately
LessThan Less Than
MoreThan More Than
<status value="active"/><experimental value="false"/><date value="2021-01-05T10:01:24+11:00"/><publisher value="HL7 (FHIR Project)"/><contact><telecom><system value="url"/><value value="http://hl7.org/fhir"/></telecom><telecom><system value="email"/><value value="fhir@lists.hl7.org"/></telecom></contact><description value="The type of a numeric quantity measurement."/><jurisdiction><coding><system value="http://unstats.un.org/unsd/methods/m49/m49.htm"/><code value="001"/><display value="World"/></coding></jurisdiction><caseSensitive value="true"/><valueSet value="http://hl7.org/fhir/ValueSet/substance-amount-type"/><content value="complete"/><concept><code value="Average"/><display value="Average"/></concept><concept><code value="Approximately"/><display value="Approximately"/></concept><concept><code value="LessThan"/><display value="Less Than"/></concept><concept><code value="MoreThan"/><display value="More Than"/></concept></CodeSystem>