How to get constraints(allowed values) of the type/field

You can view the field relationships with "im viewtype" on either command line or API.

The following is the Field Relationship section of "im viewtype" for a sample Requirement type:

CLI:

Field Relationships:
Referenced Item Type=Requirement Specification Document:Category=Document
Referenced Item Type=Shared Requirement:Category=Technical Requirement,System Requirement,Component Requirement,Heading,Business Requirement,Comment,User Requirement,Functional Requirement,Non-Functional Requirement,SRS (Software Requirement),SSS (System/Subsystem Requirement),IRS (Interface Requirement)
constraintrule=(((field["Category"] = "Non-Functional Requirement") or (field["Category"] = "Functional Requirement")) and (field["State"] = "Cancelled")):Additional Comments=:all,mandatory,description=test rule

API:

 <Field name="fieldRelationships">
 <List elementType="item">
 <Item id="Referenced Item Type" modelType="im.Field" displayId="Referenced Item Type">
 <Field name="targetFields">
 <List elementType="item">
 <Item id="Category" modelType="im.Field" displayId="Category">
 <Field name="sourceValues">
 <List>
 <Value dataType="string">Requirement Specification Document</Value>
 </List>
 </Field>
 <Field name="targetValues">
 <List>
 <Value dataType="string">Document</Value>
 </List>
 </Field>
 <Field name="targetValueType">
 <Value dataType="string">static</Value>
 </Field>
 <Field name="mandatory">
 <Value dataType="boolean">false</Value>
 </Field>
 <Field name="errMandatory">
 <Value></Value>
 </Field>
 <Field name="errInvalidated">
 <Value></Value>
 </Field>
 <Field name="description">
 <Value></Value>
 </Field>
 <Field name="cliSpec">
 <Value dataType="string">Referenced Item Type=Requirement Specification Document:Category=Document</Value>
 </Field>
 <Field name="all">
 <Value dataType="boolean">false</Value>
 </Field>
 </Item>
 <Item id="Category" modelType="im.Field" displayId="Category">
 <Field name="sourceValues">
 <List>
 <Value dataType="string">Shared Requirement</Value>
 </List>
 </Field>
 <Field name="targetValues">
 <List>
 <Value dataType="string">Technical Requirement</Value>
 <Value dataType="string">System Requirement</Value>
 <Value dataType="string">Component Requirement</Value>
 <Value dataType="string">Heading</Value>
 <Value dataType="string">Business Requirement</Value>
 <Value dataType="string">Comment</Value>
 <Value dataType="string">User Requirement</Value>
 <Value dataType="string">Functional Requirement</Value>
 <Value dataType="string">Non-Functional Requirement</Value>
 <Value dataType="string">SRS (Software Requirement)</Value>
 <Value dataType="string">SSS (System/Subsystem Requirement)</Value>
 <Value dataType="string">IRS (Interface Requirement)</Value>
 </List>
 </Field>
 <Field name="targetValueType">
 <Value dataType="string">static</Value>
 </Field>
 <Field name="mandatory">
 <Value dataType="boolean">false</Value>
 </Field>
 <Field name="errMandatory">
 <Value></Value>
 </Field>
 <Field name="errInvalidated">
 <Value></Value>
 </Field>
 <Field name="description">
 <Value></Value>
 </Field>
 <Field name="cliSpec">
 <Value dataType="string">Referenced Item Type=Shared Requirement:Category=Technical Requirement,System Requirement,Component Requirement,Heading,Business Requirement,Comment,User Requirement,Functional Requirement,Non-Functional Requirement,SRS (Software Requirement),SSS (System/Subsystem Requirement),IRS (Interface Requirement)</Value>
 </Field>
 <Field name="all">
 <Value dataType="boolean">false</Value>
 </Field>
 </Item>
 </List>
 </Field>
 </Item>
 <Item id="constraintrule=(((field[&quot;Category&quot;] = &quot;Non-Functional Requirement&quot;) or (field[&quot;Category&quot;] = &quot;Functional Requirement&quot;)) and (field[&quot;State&quot;] = &quot;Cancelled&quot;)):Additional Comments=:all,mandatory,description=test rule" modelType="ConstraintRule" displayId="constraintrule=(((field[&quot;Category&quot;] = &quot;Non-Functional Requirement&quot;) or (field[&quot;Category&quot;] = &quot;Functional Requirement&quot;)) and (field[&quot;State&quot;] = &quot;Cancelled&quot;)):Additional Comments=:all,mandatory,description=test rule">
 <Field name="targetField">
 <Item id="Additional Comments" modelType="im.Field" displayId="Additional Comments">
 </Item>
 </Field>
 <Field name="rule">
 <Value dataType="string">(((field["Category"] = "Non-Functional Requirement") or (field["Category"] = "Functional Requirement")) and (field["State"] = "Cancelled"))</Value>
 </Field>
 <Field name="mandatory">
 <Value dataType="boolean">true</Value>
 </Field>
 <Field name="errMandatory">
 <Value></Value>
 </Field>
 <Field name="errInvalidated">
 <Value></Value>
 </Field>
 <Field name="description">
 <Value dataType="string">test rule</Value>
 </Field>
 <Field name="cliSpec">
 <Value dataType="string">constraintrule=(((field["Category"] = "Non-Functional Requirement") or (field["Category"] = "Functional Requirement")) and (field["State"] = "Cancelled")):Additional Comments=:all,mandatory,description=test rule</Value>
 </Field>
 <Field name="all">
 <Value dataType="boolean">true</Value>
 </Field>
 <Field name="targetValues">
 <Value></Value>
 </Field>
 <Field name="targetValueType">
 <Value dataType="string">static</Value>
 </Field>
 </Item>
 </List>
 </Field>

You can also see all values for the Category with "im viewfield --overrideForType="[shared item type]" "Shared Category". im viewfield and im viewtype are both published API commands.