<rdgGrp>

<rdgGrp> (reading group) within a textual variation, groups two or more readings perceived to have a genetic relationship or other affinity. [12.1 The Apparatus Entry, Readings, and Witnesses]
Module textcrit — Critical Apparatus
Attributes att.global (@xml:id, @n, @xml:lang, @xml:base, @xml:space) (att.global.rendition (@rend, @style, @rendition)) (att.global.linking (@corresp, @synch, @sameAs, @copyOf, @next, @prev, @exclude, @select)) (att.global.analytic (@ana)) (att.global.facs (@facs)) (att.global.change (@change)) (att.global.responsibility (@cert, @resp)) (att.global.source (@source)) att.textCritical (@type, @cause, @varSeq, @require) (att.typed (type, @subtype)) (att.written (@hand))
Contained by
textcrit: app rdgGrp
May contain
core: note noteGrp
textcrit: lem rdg rdgGrp wit witDetail
Note

May contain readings and nested reading groups.

Usually, only one lem element should appear within a single apparatus entry, whether it appears outside a rdgGrp element or within it.
Example
<app>
 <lem wit="#El #Ra2">though</lem>
 <rdgGrp type="orthographic">
  <rdg wit="#Hg">thogh</rdg>
  <rdg wit="#La">thouhe</rdg>
 </rdgGrp>
</app>
Schematron

<sch:assert test="count(tei:lem) < 2">Only one <lem> element may appear within a <rdgGrp></sch:assert>
Content model 
<content>
 <sequence>
  <elementRef key="lem" minOccurs="0"/>
  <alternate maxOccurs="unbounded"
   minOccurs="0">
   <classRef key="model.rdgLike"/>
   <classRef key="model.noteLike"/>
   <elementRef key="witDetail"/>
   <elementRef key="wit"/>
   <elementRef key="rdgGrp"/>
  </alternate>
 </sequence>
</content>
Schema Declaration 
<rng:element name="rdgGrp">
 <rng:ref name="att.global.attributes"/>
 <rng:ref name="att.global.rendition.attributes"/>
 <rng:ref name="att.global.linking.attributes"/>
 <rng:ref name="att.global.analytic.attributes"/>
 <rng:ref name="att.global.facs.attributes"/>
 <rng:ref name="att.global.change.attributes"/>
 <rng:ref name="att.global.responsibility.attributes"/>
 <rng:ref name="att.global.source.attributes"/>
 <rng:ref name="att.textCritical.attributes"/>
 <rng:ref name="att.written.attributes"/>
 <rng:group>
  <rng:optional>
   <rng:ref name="lem"/>
  </rng:optional>
  <rng:zeroOrMore>
   <rng:choice>
    <rng:ref name="model.rdgLike"/>
    <rng:ref name="model.noteLike"/>
    <rng:ref name="witDetail"/>
    <rng:ref name="wit"/>
    <rng:ref name="rdgGrp"/>
   </rng:choice>
  </rng:zeroOrMore>
 </rng:group>
</rng:element>
element rdgGrp
{
   att.global.attributes,
   att.global.rendition.attributes,
   att.global.linking.attributes,
   att.global.analytic.attributes,
   att.global.facs.attributes,
   att.global.change.attributes,
   att.global.responsibility.attributes,
   att.global.source.attributes,
   att.textCritical.attributes,
   att.written.attributes,
   ( lem?, ( model.rdgLike | model.noteLike | witDetail | wit | rdgGrp )* )
}