Hi, i think of footprints in terms of LEF SITES. A core standard cell will have a certain height and an integer multiple of some width; or what is called in LEF as CORE SITE. Two cells would have the same footprint it they used up the same number of SITES.
You can find delay cells in your verilog, liberty, and physical libraries; often they have DEL in their names.
wht i understand is that footprint means ... cells which have same intention or purpose..... like we say ... buffer footprint then that means all the cells which can be used as buffers..... when we say delay footprint then thats mean all the cells which can be used as inverters ....when we say delay footprint then thats means all the cells which can be used as delay cells
Regarding Delay cell, depending on ur library u can have the proper nomenclature for delay cell.
Moroever, buffer can also be used as delay cell during optimization - make not be this .. manything u would find the hold fxing using small strength buffers being ussed to increase the dealy of the path..... for tool delay cells means ANY CELL which can produce the output slew to be slower compared to input slew.