In which case we are using fact less fact table?

Factless fact table are the preferred method of recording the events in the DW where there is no natural numeric measurement associated with the event. Factless fact tables also are used to gurantee coverage

There is no mesaure in Fact table

Fact less fact is used to store events like attendence, polling events etc where the need of having a fact '1' is redundant.

Fact less fact table as the name suggest is not having any fact field and only contains Dimension keys.

Those tables are used to record events.

A factless fact table is fact table that does not contain fact. They contain only dimensional keys and it captures events that happen only at information level but not included in the calculations level, just an information about an event that happen over a period. 

A factless fact table captures the many-to-many relationships between dimensions, but contains no numeric or textual facts. They are often used to record events or coverage information. Common examples of factless fact tables include:

•         Identifying product promotion events (to determine promoted products that didn’t sell)
•        Training Skill Information for Sales Representatives

We use factless fact table for tracking a process or collecting stats

1) The fact less fact table dont have any measures

2) Its widely used as a intersection table between dimensions

3) There are two types

     a) Capturing Events

     b) capturing and maintaining the  business conditions

A factless fact table is a fact table that does not have any measures. It is essentially an intersection of dimensions. On the surface, a factless fact table does not make sense, since a fact table is, after all, about facts. However, there are situations where having this kind of relationship makes sense in data warehousing

A factless fact table is a fact table that does not have any measures. It is essentially an intersection of dimensions it contains nothing but dimensional keys. There are two types of factless tables: One is for capturing an event, and one is for describing conditions.

Factless fact tables are used for tracking a process or collecting stats. There are two types of factless tables: those that describe events, and those that describe conditions.

Factless fact tables contain only dimesional keys and it captures events that happen only at information level but not included in the calculations level. 

Common uses for factless fact tables like:

Tracking student attendance or registration events.

Identifying building, facility, and equipment schedules for a hospital or university

We use factless fact table to resolve many to many cardinality issue.