Congestion

Congestion needs to be analyzed after placement and the routing results depend on how congested your design is. Routing congestion may be localized. Some of the things that you can do to make sure routing is hassle free are:

Placement blockages: 

The utilization constraint is not a hard rule, and if you want to specifically avoid placement in certain areas, use placement blockages.
Soft blockages (buffer only)
Hard blockages (No std cells and buffers are allowed to Place)
Partial blockages (same as density screens)
Halo (same as soft blockage but blockage can also be moved w.r.t Macro.)

Macro-padding:

 Macro padding or placement halos around the macros are placement blockages around the edge of the macros. This makes sure that no standard cells are placed near the pin outs of the macros, thereby giving extra breathing space for the macro pin connections to standard cells.

Cell padding:

Cell Padding refers to placement clearance applied to std cells in PnR tools. This is typically done to ease placement congestion or reserve some space for future use down the flow.
For example typically people apply cell padding to the buffers/inverters used to build clock tree, so that space is reserved to insert DECAP cells near them after CTS.

Cell padding adds hard constraints to placement. The constraints are honored by cell legalization, CTS, and timing optimization, unless the padding is reset after placement so
those operations can use the reserved space. You can use cell padding to reserve space for routing.

The command "specifyCellPad" is used to specify the cell padding in SOC-Encounter.

This command adds padding on the right side of library cells during placement.

The padding is specified in terms of a factor that is applied to the metal2 pitch. For example, if you specify a factor of 2, the software ensures that there is additional clearance of two times the metal2 pitch on the right side of the specified cells.

Maximum Utilization constraint (density screens): 

Some tools let you specify maximum core utilization numbers for specific regions. If any region has routing congestion, utilization there can be reduced, thus freeing up more area for routing.
each tool is having this setting, check wityh your DA for the detail.

set_congestion_options -max_util .6 -coordinate {10 20 40 40}