It may be easier to identify those elements that CANNOT accept a variable without a specialization. This is the list that FICO put together when we launched DITA, but it may be out of date now. Here's what our internal doc says: 

Although most DITA elements can contain the <ph> element, some cannot. Unless we decide to create a specialization, you will not be able to insert variables in the following elements. This is inconvenient because product names and our company name are frequently used in installation file paths, window titles, and menu cascades. Until we come up with a solution for this problem, you will need to type variable values
into these elements.

apiname

cmdname

filepath

kwd

menucascade

msgblock

msgnum

msgph

option

synblk

synnoteref

synph

syntaxdiagram

systemoutput

uicontrol

userinput

varname

wintitle

Not having a single element to contain variable content is annoying and something I wish had been addressed in DITA 1.3.  For most variables, <ph> works.  For variable content that has to go into places like <uicontrol>, I end up defining 2 variables with the same content: one as <ph> and one as <keyword>.

Not having a single element to contain variable content is annoying and something I wish had been addressed in DITA 1.3.  For most variables, <ph> works.  For variable content that has to go into places like <uicontrol>, I end up defining 2 variables with the same content: one as <ph> and one as <keyword>.