One of the special features of using MDX with TM1 dimensions is that existing subsets can be used within the query for defining a new subset. This can be useful in allowing a simpler ‘building block’ approach and for not having to repeat the same code over and over again – and having to maintain it.

Used throughout this section, [Report Date] is an existing subset in the Date dimension containing one leaf date member and [test2] is an existing 20-member subset.

Note that private subsets are used in preference to public subsets when there is one of each with the same name. This can allow a public subset to return different results based on the contents of different users’ private subsets, though inevitably with some issues with reliability of results.

To simply return the member(s) of pre-existing Date subsets:
[Date].[Report Date]
Or
TM1SubsetToSet([Date], "Report Date")

The first syntax may be shorter and more convenient but bear in mind, as per the TM1 help file, “Since the same syntax ( .IDENTIFIER ) is used for members and levels, a subset with the same name of a member or a level will never be instantiated.” The second syntax on the other hand will happily work with any subset names even if they are named the same as a cube or dimension.

To return the first member of the test2 subset:
{ [Date].[test2].Item(0) }

To return a valid cube reference within a more complex query:
TM1Member([Date].[Current Date].Item(0), 0)
For example:
{FILTER( {TM1FILTERBYLEVEL( {TM1SUBSETALL( [Account] )}, 1)}, [Reconciliation].([Entity].[All Entities],TM1Member([Date].[Current Date].Item(0),0),[Reconciliation Measures].[Transaction Balance]) <> 0 )}

To start with the fourth item (.Item counts from zero) in the test2 subset and then return the preceding 14 members from the whole dimension, including the fourth item:
{ lastperiods(14, tm1member( [Date].[test2].item(3),0) )}

This example returns the one date in Report Date and the next 13 periods, sorted with the earliest date first – a moving 2-week reporting window which just needs the Report Date subset to be maintained.
{ tm1sort( lastperiods(-14, TM1Member( [Date].[Report Date].Item(0), 0) ), ASC)}

And this example does a similar thing working in the other direction:
{ tm1sort( lastperiods(14, TM1Member( [Date].[Report Date].Item(0), 0) ), DESC)}

This query uses another subset, Strategic Products, as a building block and finds the Top 5 members within it, even though this ranking may well have been based on different values than the original subset was built on. For example, a subset that is already defined may list the 10 highest spending customer segments in terms of year to date actuals, and you then build a new subset that works with these 10 only to find the top 5 in terms of planned marketing spend next quarter.
{ ORDER( {TOPCOUNT( {[Product].[Strategic Products]}, 5, [Test].([Posting Measures].[Count]))}, [Test].([Posting Measures].[Count]), BDESC) }

Here’s a bigger example using TM1member and TM1SubsetToSet functions, in addition to various others. It takes the single period in the “Current Date” subset and returns the last day of the two preceding months. There would be several different ways of achieving the same result.
{
union(
{tail(descendants(head(lastperiods(3,ancestor(
tm1member(tm1subsettoset([Date], "Current Date").item(0),0),1)
))))},
{tail(descendants(head(lastperiods(2,ancestor(
tm1member(tm1subsettoset([Date], "Current Date").item(0),0),1)
))))}
)
}